Liquid Crystal Display Device and Electronic Device

ABSTRACT

To provide a circuit used for a shift register or the like. The basic configuration includes first to fourth transistors and four wirings. The power supply potential VDD is supplied to the first wiring and the power supply potential VSS is supplied to the second wiring. A binary digital signal is supplied to each of the third wiring and the fourth wiring. An H level of the digital signal is equal to the power supply potential VDD, and an L level of the digital signal is equal to the power supply potential VSS. There are four combinations of the potentials of the third wiring and the fourth wiring. Each of the first transistor to the fourth transistor can be turned off by any combination of the potentials. That is, since there is no transistor that is constantly on, deterioration of the characteristics of the transistors can be suppressed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/954,699, filed Apr. 17, 2018, now allowed, which is a continuation ofU.S. application Ser. No. 15/279,575, filed Sep. 29, 2016, now U.S. Pat.No. 9,954,010, which is a continuation of U.S. application Ser. No.14/967,458, filed Dec. 14, 2015, now U.S. Pat. No. 9,461,071, which is acontinuation of U.S. application Ser. No. 14/510,273, filed Oct. 9,2014, now U.S. Pat. No. 9,214,473, which is a continuation of U.S.application Ser. No. 13/675,066, filed Nov. 13, 2012, now U.S. Pat. No.9,070,593, which is a continuation of U.S. application Ser. No.11/747,537, filed May 11, 2007, now U.S. Pat. No. 8,330,492, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2006-155472 on Jun. 2, 2006, all of which are incorporated byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a semiconductor device. In addition,the invention relates to a display device having the semiconductordevice. In particular, the invention relates to a liquid crystal displaydevice having the semiconductor device and an electronic device havingthe liquid crystal display device.

2. Description of the Related Art

In recent years, with the increase of large display devices such asliquid crystal televisions, display devices such as liquid crystaldisplay devices and light-emitting devices have been actively developed.In particular, a technique for forming a pixel circuit and a drivercircuit including a shift register or the like (hereinafter referred toas an internal circuit) over the same substrate by using transistorsmade of an amorphous semiconductor over an insulator has been activelydeveloped, because the technique greatly contributes to low powerconsumption and low cost. The internal circuit formed over the insulatoris connected to a controller IC or the like arranged outside theinsulator (hereinafter referred to as an external circuit) through anFPC or the like, and its operation is controlled.

In addition, a shift register which is formed by using transistors madeof an amorphous semiconductor has been devised as the internal circuitformed over the insulator (see Reference 1: Japanese Published PatentApplication No. 2004-78172).

However, there has been a problem in that characteristics of transistorsformed of an amorphous semiconductor deteriorate in accordance with anon time or a voltage applied. In order to solve this problem,suppression of characteristic deterioration of the transistors has beendevised by connecting two transistors in parallel and sequentiallyturning on the transistors. (see Reference 2: SID '05 DIGEST PP. 348 toPP.351).

SUMMARY OF THE INVENTION

A detailed driving method is not disclosed in above-described Reference2. In addition, in order to control two transistors connected inparallel one by one, a control circuit having a large circuit size isnecessary.

In view of the aforementioned problems, it is an object of the inventionto provide a flip-flop circuit and a shift register each having acontrol circuit with a comparatively small circuit size, a semiconductordevice and a display device each having such a shift register, and anelectronic device having the display device.

In addition, it is another object of the invention to provide aflip-flop circuit and a shift register each using a driving method forsuppressing characteristic deterioration of a transistor which isdifferent from a conventional technique, a semiconductor device and adisplay device each having such a shift register, and an electronicdevice having the display device.

A semiconductor device in accordance with one aspect of the inventionincludes a first transistor, a second transistor, a third transistor,and a fourth transistor. A gate and a first terminal of the firsttransistor are electrically connected to a first wiring, and a secondterminal of the first transistor is electrically connected to a gate ofthe fourth transistor. A gate of the second transistor is electricallyconnected to a second wiring, a first terminal of the second transistoris electrically connected to a fourth wiring, and a second terminal ofthe second transistor is electrically connected to the gate of thefourth transistor. A gate of the third transistor is electricallyconnected to a third wiring, a first terminal of the third transistor iselectrically connected to the fourth wiring, and a second terminal ofthe third transistor is electrically connected to the gate of the fourthtransistor. A first terminal of the fourth transistor is electricallyconnected to the fourth wiring, and a second terminal of the fourthtransistor is electrically connected to a fifth wiring.

The first to fourth transistors may have the same conductivity type. Inaddition, an amorphous semiconductor may be used for a semiconductorlayer of each of the first to fourth transistors.

Note that a ratio (W/L) of channel width W to channel length L of thefirst transistor may be higher than a ratio (W/L) of channel width W tochannel length L of the second transistor.

Note that a ratio (W/L) of channel width W to channel length L of thefirst transistor may be higher than a ratio (W/L) of channel width W tochannel length L of the third transistor.

A semiconductor device in accordance with one aspect of the inventionincludes a first transistor, a second transistor, a third transistor, afourth transistor, a fifth transistor, a sixth transistor, a seventhtransistor, and an eighth transistor. A gate of the first transistor iselectrically connected to a first wiring, a first terminal of the firsttransistor is electrically connected to a second wiring, and a secondterminal of the first transistor is electrically connected to a gate ofthe second transistor. A gate of the eighth transistor is electricallyconnected to a fourth wiring, a first terminal of the eighth transistoris electrically connected to a fifth wiring, and a second terminal ofthe eighth transistor is electrically connected to the gate of thesecond transistor. A gate of the sixth transistor is electricallyconnected to the gate of the second transistor, a first terminal of thesixth transistor is electrically connected to the fifth wiring, and asecond terminal of the sixth transistor is electrically connected to agate of the third transistor and a gate of the fourth transistor. A gateand a first terminal of the fifth transistor are electrically connectedto the second wiring, and a second terminal of the fifth transistor iselectrically connected to the gate of the third transistor and the gateof the fourth transistor. A gate of the seventh transistor iselectrically connected to a third wiring, a first terminal of theseventh transistor is electrically connected to the fifth wiring, and asecond terminal of the seventh transistor is electrically connected tothe gate of the third transistor and the gate of the fourth transistor.A first terminal of the fourth transistor is electrically connected tothe fifth wiring, and a second terminal of the fourth transistor iselectrically connected to the gate of the second transistor. A firstterminal of the third transistor is electrically connected to the fifthwiring, and a second terminal of the third transistor is electricallyconnected to a sixth wiring. A first terminal of the second transistoris electrically connected to the third wiring, and a second terminal ofthe second transistor is electrically connected to the sixth wiring.

The first to eighth transistors may have the same conductivity type. Inaddition, an amorphous semiconductor may be used for a semiconductorlayer of each of the first to eighth transistors.

Note that a ratio (W/L) of channel width W to channel length L of thefifth transistor may be higher than a ratio (W/L) of channel width W tochannel length L of the sixth transistor.

Note that a ratio (W/L) of channel width W to channel length L of thefifth transistor may be higher than a ratio (W/L) of channel width W tochannel length L of the seventh transistor.

In addition, the semiconductor device of the invention may be used for aliquid crystal display device.

A liquid crystal display device in accordance with one aspect of theinvention includes a driver circuit and a pixel having a liquid crystalelement. The driver circuit includes a first transistor, a secondtransistor, a third transistor, and a fourth transistor. A gate and afirst terminal of the first transistor are electrically connected to afirst wiring, and a second terminal of the first transistor iselectrically connected to a gate of the fourth transistor. A gate of thesecond transistor is electrically connected to a second wiring, a firstterminal of the second transistor is electrically connected to a fourthwiring, and a second terminal of the second transistor is electricallyconnected to the gate of the fourth transistor. A gate of the thirdtransistor is electrically connected to a third wiring, a first terminalof the third transistor is electrically connected to the fourth wiring,and a second terminal of the third transistor is electrically connectedto the gate of the fourth transistor. A first terminal of the fourthtransistor is electrically connected to the fourth wiring, and a secondterminal of the fourth transistor is electrically connected to a fifthwiring.

The first to fourth transistors may have the same conductivity type. Inaddition, an amorphous semiconductor may be used for a semiconductorlayer of each of the first to fourth transistors.

Note that a ratio (W/L) of channel width W to channel length L of thefirst transistor may be higher than a ratio (W/L) of channel width W tochannel length L of the second transistor.

Note that a ratio (W/L) of channel width W to channel length L of thefirst transistor may be higher than a ratio (W/L) of channel width W tochannel length L of the third transistor.

A liquid crystal display device in accordance with one aspect of theinvention includes a driver circuit and a pixel having a liquid crystalelement. The driver circuit includes a first transistor, a secondtransistor, a third transistor, a fourth transistor, a fifth transistor,a sixth transistor, a seventh transistor, and an eighth transistor. Agate of the first transistor is electrically connected to a firstwiring, a first terminal of the first transistor is electricallyconnected to a second wiring, and a second terminal of the firsttransistor is electrically connected to a gate of the second transistor.A gate of the eighth transistor is electrically connected to a fourthwiring, a first terminal of the eighth transistor is electricallyconnected to a fifth wiring, and a second terminal of the eighthtransistor is electrically connected to the gate of the secondtransistor. A gate of the sixth transistor is electrically connected tothe gate of the second transistor, a first terminal of the sixthtransistor is electrically connected to the fifth wiring, and a secondterminal of the sixth transistor is electrically connected to a gate ofthe third transistor and a gate of the fourth transistor. A gate and afirst kLOuial of the fifth transistor are electrically connected to thesecond wiring, and a second terminal of the fifth transistor iselectrically connected to the gate of the third transistor and the gateof the fourth transistor. A gate of the seventh transistor iselectrically connected to a third wiring, a first terminal of theseventh transistor is electrically connected to the fifth wiring, and asecond terminal of the seventh transistor is electrically connected tothe gate of the third transistor and the gate of the fourth transistor.A first terminal of the fourth transistor is electrically connected tothe fifth wiring, and a second terminal of the fourth transistor iselectrically connected to the gate of the second transistor. A firstterminal of the third transistor is electrically connected to the fifthwiring, and a second terminal of the third transistor is electricallyconnected to a sixth wiring. A first terminal of the second transistoris electrically connected to the third wiring, and a second terminal ofthe second transistor is electrically connected to the sixth wiring.

The first to eighth transistors may have the same conductivity type. Inaddition, an amorphous semiconductor may be used for a semiconductorlayer of each of the first to eighth transistors.

Note that a ratio (W/L) of channel width W to channel length L of thefifth transistor may be higher than a ratio (W/L) of channel width W tochannel length L of the sixth transistor.

Note that a ratio (W/L) of channel width W to channel length L of thefifth transistor may be higher than a ratio (W/L) of channel width W tochannel length L of the seventh transistor.

Note that various types of switches can be used as a switch shown in theinvention, and an electrical switch, a mechanical switch, and the likeare given as examples. That is, any element can be used as long as itcan control a current flow, without limiting to a certain element. Forexample, it may be a transistor, a diode (e.g., a PN diode, a PIN diode,a Schottky diode, or a diode-connected transistor), a thyristor, or alogic circuit combining such elements. In the case of using a transistoras a switch, the polarity (the conductivity type) of the transistor isnot particularly limited to a certain type because it operates just as aswitch. However, a transistor of polarity with smaller off-current ispreferably used when off-current is preferably small. A transistorprovided with an LDD region, a transistor with a multi-gate structure,and the like are given as examples of a transistor with smalleroff-current. In addition, it is preferable that an N-channel transistorbe used when a potential of a source terminal of the transistor which isoperated as a switch is closer to a low-potential-side power supply(e.g., Vss, GND, or 0 V), while a P-channel transistor be used when thepotential of the source terminal is closer to a high-potential-sidepower supply (e.g., Vdd). This is because the absolute value of agate-source voltage of the transistor is increased, so that thetransistor can easily operate as a switch.

A CMOS switch may also be employed by using both N-channel and P-channeltransistors. By employing the CMOS switch, the switch can efficientlyoperate as a switch since a current can flow through the switch when oneof the P-channel switch and the N-channel switch is turned on. Forexample, a voltage can be appropriately output regardless of whether avoltage of an input signal of the switch is high or low. In addition,since a voltage amplitude value of a signal for turning on or off theswitch can be made small, power consumption can be reduced.

When a transistor is employed as a switch, the switch includes an inputterminal (one of a source terminal and a drain terminal), an outputterminal (the other of the source terminal and the drain terminal), anda terminal for controlling electrical conduction (a gate terminal). Onthe other hand, when a diode is employed as a switch, the switch doesnot have a terminal for controlling electrical conduction in some cases.Therefore, the number of wirings for controlling terminals can bereduced.

Note that in the invention, the description “being connected” includesthe case where elements are electrically connected, the case whereelements are functionally connected, and the case where elements aredirectly connected. Accordingly, in the configurations disclosed in theinvention, other elements may be interposed between elements having apredetermined connection relation. For example, one or more elementswhich enable electrical connection (e.g., a switch, a transistor, acapacitor, an inductor, a resistor, and/or a diode) may be providedbetween a certain portion and another portion. In addition, one or morecircuits which enable functional connection may be provided between theportions, such as a logic circuit (e.g., an inverter, a NAND circuit, ora NOR circuit), n signal converter circuit (e.g., a DA convertercircuit, an AD converter circuit, or a gamma correction circuit), apotential level converter circuit (e.g., a power supply circuit such asa boosting circuit or a voltage lower control circuit, or a levelshifter circuit for changing a potential level of an H-level signal oran L-level signal), a voltage source, a current source, a switchingcircuit, or an amplifier circuit (e.g., a circuit which can increase thesignal amplitude, the amount of current, or the like, such as anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit), a signal generating circuit, amemory circuit, or a control circuit. Alternatively, the elements may bedirectly connected without interposing another element or anothercircuit therebetween.

In the case where elements are connected without interposing anotherelement or circuit therebetween, the description “being directlyconnected” is employed. In addition, in the case where the description“being electrically connected” is employed, the following cases areincluded therein: the case where elements are electrically connected(that is, the case where the elements are connected by interposinganother element therebetween), the case where elements are functionallyconnected (that is, the elements are connected by interposing anothercircuit therebetween), and the case where elements are directlyconnected (that is, the elements are connected without interposinganother element or another circuit therebetween).

Note that a display element, a display device, a light-emitting element,and a light-emitting device can apply various types and include variouselements. For example, as a display element, a display device, alight-emitting element, and a light-emitting device, a display medium,the contrast of which changes by an electromagnetic action, such as anEL element (e.g., an organic EL element, an inorganic EL element, or anEL element including both organic and inorganic materials) anelectron-emissive element, a liquid crystal, electronic ink, a gratinglight valve (GLV), a plasma display panel (PDP), a digital micromirrordevice (DMD), a piezoelectric ceramic display, or a carbon nanotube canbe applied. Note that display devices using an EL element include an ELdisplay; display devices using an electron-emissive element include afield emission display (FED), an SED-type flat panel display (SED:Surface-conduction Electron-emitter Display), and the like; displaydevices using a liquid crystal element include a liquid crystal display,a transmissive liquid crystal display, a semi-transmissive liquidcrystal display, a reflective liquid crystal display, and the like; anddisplay devices using electronic ink include electronic paper.

Note that in the invention, various types of transistors can be employedas a transistor without limiting to a certain type. Thus, for example, athin film transistor (TFT) including a non-single crystallinesemiconductor film typified by amorphous silicon or polycrystallinesilicon can be employed. Accordingly, such a transistor can be formed atlow temperature, can be formed at low cost, can be formed over a largesubstrate as well as a light-transmissive substrate, and further, such atransistor can transmit light. In addition, a transistor formed by usinga semiconductor substrate or an SOI substrate, a MOS transistor, ajunction transistor, a bipolar transistor, or the like can be employed.Accordingly, a transistor with few variations, a transistor with highcurrent supply capacity, and a transistor with a small size can beformed, thereby a circuit with low power consumption can be formed byusing such a transistor. In addition, a transistor including a compoundsemiconductor such as ZnO, a-InGaZnO, SiGe, or GaAs, or a thin filmtransistor obtained by thinning such a compound semiconductor can beemployed. Therefore, such a transistor can be formed at low temperature,can be formed at room temperature, and can be formed directly over a lowheat-resistant substrate such as a plastic substrate or a filmsubstrate. A transistor or the like formed by an inkjet method or aprinting method may also be employed. Accordingly, such a transistor canbe formed at room temperature, can be formed at a low vacuum, or can beformed using a large substrate. In addition, since such a transistor canbe formed without using a mask (a reticle), layout of the transistor canbe easily changed. Further, a transistor including an organicsemiconductor or a carbon nanotube, or other transistors can beemployed. Accordingly, the transistor can be formed using a substratewhich can be bent. Note that a non-single crystalline semiconductor filmmay include hydrogen or halogen. Moreover, a transistor can be formedusing various types of substrates. The type of a substrate is notlimited to a certain type. Therefore, for example, a single crystallinesubstrate, an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a paper substrate, a cellophane substrate, a stonesubstrate, a stainless steel substrate, a substrate including astainless steel foil, or the like can be used as a substrate.Furthermore, the transistor may be formed using one substrate, and then,the transistor may be transferred to another substrate. By using theaforementioned substrate, a transistor with excellent properties or atransistor with low power consumption can be formed, or a device withhigh durability or high heat resistance can be formed.

The structure of a transistor can be various modes without limiting to acertain structure. For example, a multi-gate structure having two ormore gate electrodes may be used. When the multi-gate structure is used,a structure where a plurality of transistors are connected in series isprovided because a structure whlse channel regions are connected inseries is provided. By using the multi-gate structure, off-current canbe reduced; the withstand voltage of the transistor can be increased toimprove reliability; or a drain-source current does not fluctuate verymuch even if a drain-source voltage fluctuates when the transistoroperates in the saturation region so that flat characteristics can beobtained. In addition, a structure where gate electrodes are formedabove and below a channel may be used. By using the structure where gateelectrodes are formed above and below the channel, a channel region isenlarged to increase the amount of a current flowing therethrough, or adepletion layer can be easily formed to decrease the S value. When thegate electrodes are formed above and below the channel, a structurewhere a plurality of transistors are connected in parallel is provided.Further, a structure where a gate electrode is formed above a channel, astructure where a gate electrode is formed below a channel, a staggeredstructure, or an inversely staggered structure may be used; or a channelregion may be divided into a plurality of regions and the dividedregions may be connected in parallel or in series. A source electrode ora drain electrode may overlap with a channel (or a part of it). By usingthe structure where the source electrode or the drain electrode mayoverlap with the channel (or a part of it), the case can be prevented inwhich electric charges are accumulated in a part of the channel, whichwould result in an unstable operation. Moreover, an LDD region may beprovided. By providing the LDD region, off-current can be reduced; thewithstand voltage of the transistor can be increased to improvereliability; or a drain-source current does not fluctuate very much evenif a drain-source voltage fluctuates when the transistor operates in thesaturation region so that flat characteristics can be obtained.

Note that various types of transistors can be used for a transistor inthe invention and the transistor can be formed using various types ofsubstrates. Accordingly, all of circuits may be formed using a glasssubstrate, a plastic substrate, a single crystalline substrate, an SOTsubstrate, or any other substrate. When all of the circuits are formedusing the same substrate, the number of component parts can be reducedto cut cost, or the number of connections between circuit components canbe reduced to improve reliability. Alternatively, a part of the circuitsmay be formed using one substrate and another part of the circuits maybe formed using another substrate. That is, not all of the circuits arerequired to be formed using the same substrate. For example, a part ofthe circuits may be formed with transistors using a glass substrate andanother part of the circuits may be formed using a single crystallinesubstrate, so that the IC chip may be connected to the glass substrateby COG (Chip On Glass). Alternatively, the IC chip may be connected tothe glass substrate by TAB (Tape Automated Bonding) or a printed wiringboard. When a part of the circuits is formed using the same substrate inthis manner, the number of the component parts can be reduced to cutcost, or the number of connections between the circuit components can bereduced to improve reliability. In addition, by forming a portion with ahigh driving voltage or a portion with high driving frequency, whichconsumes large power, over another substrate, increase in powerconsumption can be prevented.

Note also that one pixel corresponds to one element whose brightness canbe controlled in the invention. Therefore, for example, one pixelcorresponds to one color element and brightness is expressed with theone color element. Accordingly, in the case of a color display devicehaving color elements of R (Red), G (Green), and B (Blue), a minimumunit of an image is formed of three pixels of an R pixel, a G pixel, anda B pixel. Note that the color elements are not limited to three colors,and color elements of more than three colors may be used or a colorother than RGB may be added. For example, RGBW (W means white) may beused by adding white. In addition, RGB plus one or more colors ofyellow, cyan, magenta emerald green, vermilion, and the like may beused. Further, a color similar to at least one of R, G, and B may beadded. For example, R, QG B1, and B2 may be used. Although both B1 andB2 are blue, they have slightly different frequency. By using such colorelements, display which is closer to the real object can be performed orpower consumption can be reduced. Alternatively, as another example, inthe case of controlling brightness of one color element by using aplurality of regions, one region corresponds to one pixel. Therefore,for example, in the case of performing area gray scale display, aplurality of regions which control brightness are provided in each colorelement and gray scales are expressed with the whole regions. In thiscase, one region which controls brightness corresponds to one pixel.Thus, in that case, one color element includes a plurality of pixels.Further, in that case, regions which contribute to display may havedifferent area dimensions depending on pixels. Moreover, in a pluralityof regions which control brightness in each color element, that is, in aplurality of pixels which form one color element, signals supplied tothe plurality of the pixels may be slightly varied so that the viewingangle can be widened. Note that the description “one pixel (for threecolors)” corresponds to the case where three pixels of R, G, and B areconsidered as one pixel. Meanwhile, the description “one pixel (for onecolor)” corresponds to the case where a plurality of pixels are providedin each color element and collectively considered as one pixel.

Note also that in the invention, pixels may be provided (arranged) inmatrix. Here, description that pixels are provided (arranged) in matrixincludes the case where the pixels are arranged in a straight line andthe case where the pixels are arranged in a jagged line, in alongitudinal direction or a lateral direction. Therefore, in the case ofperforming full color display with three color elements (e.g., RGB), thefollowing cases are included therein: the case where the pixels arearranged in stripes and the case where dots of the three color elementsare arranged in a so-called delta pattern. In addition, the case is alsoincluded therein in which dots of the three color elements are providedin Bayer arrangement. Note that the color elements are not limited tothree colors, and color elements of more than three colors may beemployed. RGBW (W means white), RGB plus one or more of yellow, cyan,magenta, and the like, or the like is given as an example. Further, thesizes of display regions may be different between respective dots ofcolor elements. Thus, power consumption can be reduced or the life of alight-emitting element can be prolonged.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor has a channel regionbetween a drain region and a source region, and a current can flowthrough the drain region, the channel region, and the source region.Here, since the source and the drain of the transistor may changedepending on the structure, the operating condition, and the like of thetransistor, it is difficult to define which is a source or a drain.Therefore, in the invention, a region functioning as a source and adrain may not be called the source or the drain. In such a case, forexample, one of the source and the drain may be called a first terminaland the other thereof may be called a second terminal.

Note also that a transistor may be an element having at least threeterminals of a base, an emitter, and a collector. In this case also, oneof the emitter and the collector may be similarly called a firstterminal and the other terminal may be called a second terminal.

A gate means all of or a part of a gate electrode and a gate wiring(also called a gate line, a gate signal line, or the like). A gateelectrode means a conductive film which overlaps with a semiconductorwhich forms a channel region, an LDD (Lightly Doped Drain) region, orthe like with a gate insulating film interposed therebetween. Agatewiring means a wiring for connecting a gate electrode of each pixel toeach other, or a wiring for connecting a gate electrode to anotherwiring.

However, there is a portion which functions as both a gate electrode anda gate wiring. Such a region may be called either a gate electrode or agate wiring. That is, there is a region where a gate electrode and agate wiring cannot be clearly distinguished from each other. Forexample, in the case where a channel region overlaps with an extendedgate wiring, the overlapped region functions as both a gate wiring and agate electrode. Accordingly, such a region may be called either a gateelectrode or a gate wiring.

In addition, a region formed of the same material as a gate electrodeand connected to the gate electrode may also be called a gate electrode.Similarly, a region formed of the same material as a gate wiring andconnected to the gate wiring may also be called a gate wiring. In astrict sense, such a region does not overlap with a channel region, ordoes not have a function of connecting the gate electrode to anothergate electrode in some cases. However, there is a region formed of thesame material as the gate electrode or the gate wiring and connected tothe gate electrode or the gate wiring because of the manufacturingcondition or the like. Accordingly, such a region may also be calledeither a gate electrode or a gate wiring.

In a multi-gate transistor, for example, a gate electrode of onetransistor is often connected to a gate electrode of another transistorby using a conductive film which is formed of the same material as thegate electrode. Since such a region is a region for connecting the gateelectrode to another gate electrode, it may be called a gate wiring, andit may also be called a gate electrode because a multi-gate transistorcan be considered as one transistor. That is, a region which is formedof the same material as the gate electrode or the gate wiring andconnected thereto may be called either a gate electrode or a gatewiring. In addition, for example, a part of a conductive film whichconnects the gate electrode and the gate wiring may also be calledeither a gate electrode or a gate wiring.

Note that a gate terminal means a part of a region of a gate electrodeor a part of a region which is electrically connected to the gateelectrode.

Note also that a source means all of or a part of a source region, asource electrode, and a source wiring (also called a source line, asource signal line, or the like). A source region means a semiconductorregion containing a large amount of P-type impurities (e.g., boron orgallium) or N-type impurities (e.g., phosphorus or arsenic).Accordingly, a region containing a small amount of P-type impurities orN-type impurities, namely, an LDD (Lightly Doped Drain) region is notincluded in the source region. A source electrode is a part of aconductive layer formed of a material different from that of a sourceregion, and electrically connected to the source region. However, thereis the case where a source electrode and a source region arecollectively called a source electrode. A source wiring is a wiring forconnecting a source electrode of each pixel to each other, or a wiringfor connecting a source electrode to another wiring.

However, there is a portion functioning as both a source electrode and asource wiring. Such a region may be called either a source electrode ora source wiring. That is, there is a region where a source electrode anda source wiring cannot be clearly distinguished from each other. Forexample, in the case where a source region overlaps with an extendedsource wiring, the overlapped region functions as both a source wiringand a source electrode. Accordingly, such a region may be called eithera source electrode or a source wiring.

In addition, a region formed of the same material as a source electrodeand connected to the source electrode, or a portion for connecting asource electrode to another source electrode may also be called a sourceelectrode. A portion which overlaps with a source region may also becalled a source electrode. Similarly, a region formed of the samematerial as a source wiring and connected to the source wiring may becalled a source wiring. In a strict sense, such a region may not have afunction of connecting the source electrode to another source electrode.However, there is a region formed of the same material as the sourceelectrode or the source wiring, and connected to the source electrode orthe source wiring because of the manufacturing condition or the like.Accordingly, such a region may also be called either a source electrodeor a source wiring.

In addition, for example, a part of a conductive film which connects asource electrode and a source wiring may be called either a sourceelectrode or a source wiring.

Note that a source terminal means a part of a source region, a part of asource electrode, or a part of a region electrically connected to thesource electrode.

Note also that the same can be said for a drain.

In the invention, a semiconductor device means a device having a circuitincluding a semiconductor element (e.g., a transistor or a diode). Thesemiconductor device may also include all devices that can function byutilizing semiconductor characteristics.

In addition, a display device means a device having a display element(e.g., a liquid crystal element or a light-emitting element). Note thatthe display device may also means a display panel itself where aplurality of pixels including display elements such as liquid crystalelements or LEL elements are formed over the same substrate as aperipheral driver circuit for driving the pixels. In addition, thedisplay device may also include a peripheral driver circuit providedover a substrate by wire bonding or bump bonding, namely, chip on glass(COG). Further, the display device may also include a flexible printedcircuit (FPC) or a printed wiring board (PWB) attached to the displaypanel (e.g., an IC, a resistor, a capacitor, an inductor, or atransistor). The display device may also include an optical sheet suchas a polarizing plate or a retardation plate. Moreover, the displaydevice may include a backlight unit (a light guide plate, a prism sheet,a diffusion sheet, a reflective sheet, or a light source (e.g., an LEDor a cold cathode tube)).

In addition, a light-emitting device means a display device having aself-luminous display element, particularly, such as an EL element or anelement used for an FED. A liquid crystal display device means a displaydevice having a liquid crystal element.

In the invention, description that an object is “formed on” or “formedover” another object does not necessarily mean that the object is indirect contact with another object. The description includes the casewhere two objects are not in direct contact with each other, that is,the case where another object is interposed therebetween. Accordingly,for example, when it is described that a layer B is formed on (or over)a layer A, it includes both of the case where the layer B is formed indirect contact with the layer A, and the case where another layer (e.g.,a layer C or a layer D) is formed in direct contact with the layer A andthe layer B is formed in direct contact with the layer C or D.Similarly, when it is described that an object is formed above anotherobject, it does not necessarily mean that the object is in directcontact with another object, and another object may be interposedtherebetween. Accordingly, for example, when it is described that alayer B is formed above a layer A, it includes both of the case wherethe layer B is formed in direct contact with the layer A, and the casewhere another layer (e.g., a layer C or a layer D) is formed in directcontact with the layer A and the layer B is formed in direct contactwith the layer C or D. Similarly, when it is described that an object isformed below or under another object, it includes both of the case wherethe objects are in direct contact with each other, and the case wherethe objects are not in contact with each other.

By using the invention, a flip-flop circuit and a shift register eachusing a driving method for suppressing characteristic deterioration of atransistor, a semiconductor device and a display device each having sucha shift register, and an electronic device having the display device canbe provided.

For example, in the case of applying the invention to a shift register,because a transistor which supplies a power supply potential to anoutput terminal is not always on in a non-selection period,characteristics deterioration (e.g., a threshold potential shift) of thetransistor can be suppressed. Therefore, a malfunction of the shiftregister due to the characteristic deterioration can be suppressed.

In addition, by using the invention, a flip-flop circuit and a shiftregister each having a control circuit with a comparatively smallcircuit size, a semiconductor device and a display device each havingsuch a shift register, and an electronic device having the displaydevice can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B illustrate Embodiment Mode 1;

FIGS. 2A and 2B illustrate Embodiment Mode 1;

FIGS. 3A and 3B illustrate Embodiment Mode 1;

FIGS. 4A and 4B illustrate Embodiment Mode 1;

FIGS. 5A and 5B illustrate Embodiment Mode 2;

FIGS. 6A and 6B illustrate Embodiment Mode 2;

FIGS. 7A and 7B illustrate Embodiment Mode 2;

FIGS. 8A and 8B illustrate Embodiment Mode 2;

FIGS. 9A and 9B illustrate Embodiment Mode 3;

FIGS. 10A and 10B illustrate Embodiment Mode 3;

FIGS. 11A and 11B illustrate Embodiment Mode 3;

FIGS. 12A and 12B illustrate Embodiment Mode 3;

FIGS. 13A and 13B illustrate Embodiment Mode 1;

FIGS. 14A and 14B illustrate Embodiment Mode 1;

FIGS. 15A and 15B illustrate Embodiment Mode 1;

FIGS. 16A and 16B illustrate Embodiment Mode 1;

FIGS. 17A and 17B illustrate Embodiment Mode 2;

FIGS. 18A and 18B illustrate Embodiment Mode 2;

FIGS. 19A and 19B illustrate Embodiment Mode 2;

FIGS. 20A and 20B illustrate Embodiment Mode 2;

FIGS. 21A and 21B illustrate Embodiment Mode 3;

FIGS. 22A and 22B illustrate Embodiment Mode 3;

FIGS. 23A and 23B illustrate Embodiment Mode 3;

FIGS. 24A and 24B illustrate Embodiment Mode 3;

FIGS. 25A and 25B illustrate Embodiment Mode 4;

FIGS. 26A and 26B illustrate Embodiment Mode 4;

FIG. 27 illustrates Embodiment Mode 5;

FIG. 28 illustrates Embodiment Mode 5;

FIG. 29 illustrates Embodiment Mode 5;

FIG. 30 illustrates Embodiment Mode 5;

FIG. 31 illustrates Embodiment Mode 5;

FIG. 32 illustrates Embodiment Mode 5;

FIG. 33 illustrates Embodiment Mode 5;

FIG. 34 illustrates Embodiment Mode 5;

FIG. 35 illustrates Embodiment Mode 5;

FIG. 36 illustrates Embodiment Mode 6;

FIG. 37 illustrates Embodiment Mode 6;

FIG. 38 illustrates Embodiment Mode 6;

FIG. 39 illustrates Embodiment Mode 6;

FIG. 40 illustrates Embodiment Mode 6;

FIGS. 41A and 41B illustrate Embodiment Mode 23;

FIG. 42 illustrates Embodiment Mode 23;

FIGS. 43A and 43B illustrate Embodiment Mode 23;

FIG. 44 illustrates Embodiment Mode 5;

FIG. 45 illustrates Embodiment Mode 5;

FIG. 46 illustrates Embodiment Mode 5;

FIG. 47 illustrates Embodiment Mode 5;

FIG. 48 illustrates Embodiment Mode 6;

FIG. 49 illustrates Embodiment Mode 6;

FIG. 50 illustrates Embodiment Mode 6;

FIG. 51 illustrates Embodiment Mode 6;

FIG. 52 illustrates Embodiment Mode 6;

FIG. 53 illustrates Embodiment Mode 23;

FIG. 54 illustrates Embodiment Mode 23;

FIG. 55 illustrates Embodiment Mode 23;

FIG. 56 illustrates Embodiment Mode 7;

FIG. 57 illustrates Embodiment Mode 7;

FIG. 58 illustrates Embodiment Mode 7;

FIG. 59 illustrates Embodiment Mode 7;

FIG. 60 illustrates Embodiment Mode 8;

FIG. 61 illustrates Embodiment Mode 8;

FIG. 62 illustrates Embodiment Mode 9;

FIG. 63 illustrates Embodiment Mode 9;

FIG. 64 illustrates Embodiment Mode 9;

FIG. 65 illustrates Embodiment Mode 10;

FIG. 66 illustrates Embodiment Mode 10;

FIGS. 67A and 67B illustrate Embodiment Mode 15;

FIG. 68 illustrates Embodiment Mode 16;

FIGS. 69A and 69B illustrate Embodiment Mode 17;

FIGS. 70A to 70C illustrate Embodiment Mode 18;

FIGS. 71A and 71B illustrate Embodiment Mode 19;

FIGS. 72A to 72C illustrate Embodiment Mode 20;

FIG. 73 illustrates Embodiment Mode 21;

FIGS. 74A to 74D illustrate Embodiment Mode 22;

FIGS. 75A and 75B illustrate Embodiment Mode 11;

FIGS. 76A and 76B illustrate Embodiment Mode 12;

FIGS. 77A to 77C illustrate Embodiment Mode 13; and

FIGS. 78A and 78B illustrate Embodiment Mode 14.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the invention will be described by way of embodiment modeswith reference to the drawings. However, the invention can beimplemented by various different ways and it will be easily understoodby those skilled in the art that various changes and modifications arepossible. Unless such changes and modifications depart from the spiritand the scope of the invention, they should be construed as beingincluded therein. Therefore, the invention should not be construed asbeing limited to the description of the embodiment modes.

Embodiment Mode 1

In this embodiment mode, a basic principle of the invention is describedwith reference to FIG. 1A.

FIG. 1A shows a basic circuit which is based on the basic principle ofthe invention. The basic circuit in FIG. 1A includes a transistor 101, atransistor 102, a transistor 103, and a transistor 104.

Connection relations of the basic circuit in FIG. 1A are described. Agate of the transistor 101 is connected to a wiring 105, a firstterminal of the transistor 101 is connected to the wiring 105, and asecond terminal of the transistor 101 is connected to a gate of thetransistor 104. A gate of the transistor 102 is connected to a wiring107, a first terminal of the transistor 102 is connected to a wiring106, and a second terminal of the transistor 102 is connected to thegate of the transistor 104. A gate of the transistor 103 is connected toa wiring 108, a first terminal of the transistor 103 is connected to thewiring 106, and a second terminal of the transistor 103 is connected tothe gate of the transistor 104. A first terminal of the transistor 104is connected to the wiring 106, and a second terminal of the transistor104 is connected to a wiring 109. Note that a node of the secondterminal of the transistor 101, the second terminal of the transistor102, the second terminal of the transistor 103, and the gate of thetransistor 104 is denoted by N11.

In addition, each of the transistors 101 to 104 is an N-channeltransistor.

Accordingly, since the basic circuit in FIG. 1A can be formed by usingonly N-channel transistors, amorphous silicon can be used for asemiconductor layer of the basic circuit in FIG. 1A. Thus, amanufacturing process can be simplified, so that manufacturing cost canbe reduced and a yield can be improved. In addition, a semiconductordevice such as a large display panel can also be formed. Further, whenpolysilicon or single crystalline silicon is used for the semiconductorlayer of the basic circuit in FIG. 1A, the manufacturing process canalso be simplified.

In addition, a power supply potential VDD is supplied to the wiring 105and a power supply potential VSS is supplied to the wiring 106. Notethat the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 105 and the wiring 106, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 107 and thewiring 108. Note that the signal supplied to each of the wiring 107 andthe wiring 108 is a binary digital signal. When the digital signal is anH-level signal, it has the same potential as the power supply potentialVDD (hereinafter also referred to as a potential VDD or an H level), andwhen the digital signal is an L-level signal, it has the same potentialas the power supply potential VSS (hereinafter also referred to as apotential VSS or an L level). Note that the power supply potential VDD,the power supply potential VSS, or another power supply potential may besupplied to each of the wiring 107 and the wiring 108. Alternatively, ananalog signal may be supplied to each of the wiring 107 and the wiring108.

Next, operations of the basic circuit shown in FIG. 1A are describedwith reference to FIG. 1B.

FIG. 1B is an example of a timing chart of the basic circuit shown inFIG. 1A. The timing chart in FIG. 1B shows a potential of the wiring107, a potential of the wiring 108, a potential of the node N11, apotential of the wiring 109, and on/off of the transistor 104.

The timing chart in FIG. 1B is described by dividing the whole periodinto periods T1 to T4. In addition, FIGS. 2A to 3B show operations ofthe basic circuit in FIG. 1A in the periods T1 to T4, respectively.

First, the operation in the period T1 is described with reference toFIG. 2A. In the period T1, an L-level signal is supplied to the wiring107 and an L-level signal is supplied to the wiring 108. Accordingly,the transistor 102 is turned off and the transistor 103 is off.

In addition, since the transistor 101 is diode-connected, the potentialof the node N11 starts to rise. This rise in the potential of the nodeN11 continues until the transistor 101 is turned off. The transistor 101is turned off when the potential of the node N1 becomes a value obtainedby subtracting a threshold voltage Vth101 of the transistor 101 from thepower supply potential VDD (VDD−Vth101). Therefore, the potential of thenode N11 becomes VDD-Vth101.

Accordingly, the transistor 104 is turned on and the potential of thewiring 109 becomes equal to the power supply potential VSS.

Next, the operation in the period T2 is described with reference to FIG.2B. In the period T2, an H-level signal is supplied to the wiring 107and an L-level signal is supplied to the wiring 108. Accordingly, thetransistor 102 is turned on and the transistor 103 is off.

In addition, the potential of the node N11 is determined by theoperating point of the transistor 101 and the transistor 102. Note thatwhen a ratio (W/L) of the transistor 102 (W means channel width of achannel region and L means channel length of the channel region) is setsufficiently higher than a ratio (W/L) of the transistor 101, thepotential of the node N11 becomes slightly higher than the power supplypotential VSS. Accordingly, the transistor 104 is turned off and thewiring 109 becomes a floating state. The potential of the wiring 109remains equal to the power supply potential VSS because the wiring 109is kept at the potential in the period T1.

Next, the operation in the period T3 is described with reference to FIG.3A. In the period T3, an L-level signal is supplied to the wiring 107and an H-level signal is supplied to the wiring 108. Accordingly, thetransistor 102 is turned off and the transistor 103 is on.

In addition, the potential of the node N11 is determined by theoperating point of the transistor 101 and the transistor 103. Note thatwhen a ratio (W/L) of the transistor 103 is set sufficiently higher thana ratio (W/L) of the transistor 101, the potential of the node N11becomes slightly higher than the power supply potential VSS.

Accordingly, the transistor 104 is turned off and the wiring 109 becomesa floating state. The potential of the wiring 109 remains equal to thepower supply potential VSS because the wiring 109 is kept at thepotential in the periods T1 and T2.

Next, the operation in the period T4 is described with reference to FIG.3B. In the period T4, an H-level signal is supplied to the wiring 107and an H-level signal is supplied to the wiring 108. Accordingly, thetransistor 102 is turned on and the transistor 104 is on.

In addition, since the potential of the node N11 is determined by theoperating point of the transistor 101, the transistor 102, and thetransistor 103, the potential of the node N11 becomes slightly higherthan the power supply potential VSS.

Accordingly, the transistor 104 is turned off and the wiring 109 becomesa floating state. The potential of the wiring 109 remains equal to thepower supply potential VSS because the wiring 109 is kept at thepotential in the periods T1 to T3.

By the above-described operations, the basic circuit in FIG. 1A suppliesthe power supply potential VSS to the wiring 109 in the period T1, sothat the potential of the wiring 109 becomes equal to the power supplypotential VSS. In the periods T2 to T4, the basic circuit in FIG. 1Amakes the wiring 109 into a floating state, so that the potential of thewiring 109 is kept equal to the power supply potential VSS.

In addition, the basic circuit in FIG. 1A does not include a transistorwhich is on in all of the periods T1 to T4. That is, the basic circuitin FIG. 1A does not include a transistor which is always or almostalways on. Accordingly, the basic circuit in FIG. 1A can suppresscharacteristic deterioration of a transistor and a threshold voltageshift due to the characteristic deterioration.

Further, the characteristics of a transistor which is formed ofamorphous silicon easily deteriorate. Therefore, when the transistorincluded in the basic circuit in FIG. 1A is formed using amorphoussilicon, not only can the advantages such as a reduction inmanufacturing cost and improvement in a yield be obtained, but also theproblem of the characteristic deterioration of the transistor can besolved.

Here, the functions of the transistors 101 to 104 are described. Thetransistor 101 has a function of a diode in which the first terminal andthe gate correspond to an input terminal and the second terminalcorresponds to an output terminal. The transistor 102 has a function ofa switch which selects whether to connect the wiring 106 and the nodeN11 in accordance with the potential of the wiring 107. The transistor103 has a function of a switch which selects whether to connect thewiring 106 and the node N11 in accordance with the potential of thewiring 108. The transistor 104 has a function of a switch which selectswhether to connect the wiring 106 and the wiring 109 in accordance withthe potential of the node N11.

Note that the transistor 101 may be any element as long as it has aresistance component. For example, as shown in FIG. 4A, a resistor 401can be used instead of the transistor 101. By using the resistor 401,the potential of the node N11 can be set equal to the power supplypotential VDD in the period T1. In addition, a timing chart in FIG. 4Ais shown in FIG. 4B.

Next, the case is described in which the basic circuit shown in FIG. 1Ais constructed from P-channel transistors, with reference to FIG. 13A.

FIG. 13A shows a basic circuit which is based on the basic principle ofthe invention. The basic circuit in FIG. 13A includes a transistor 1301,a transistor 1302, a transistor 1303, and a transistor 1304.

Connection relations of the basic circuit in FIG. 13A are described. Agate of the transistor 1301 is connected to a wiring 1306, a firstterminal of the transistor 1301 is connected to the wiring 1306, and asecond terminal of the transistor 1301 is connected to a gate of thetransistor 1304. A gate of the transistor 1302 is connected to a wiring1307, a first terminal of the transistor 1302 is connected to a wiring1305, and a second terminal of the transistor 1302 is connected to thegate of the transistor 1304. A gate of the transistor 1303 is connectedto a wiring 1308, a first terminal of the transistor 1303 is connectedto the wiring 1305, and a second terminal of the transistor 1303 isconnected to the gate of the transistor 1304. A first terminal of thetransistor 1304 is connected to the wiring 1305, and a second terminalof the transistor 1304 is connected to a wiring 1309. Note that a nodeof the second terminal of the transistor 1301, the second terminal ofthe transistor 1302, the second terminal of the transistor 1303, and thegate of the transistor 1304 is denoted by N131.

In addition, each of the transistors 1301 to 1304 is a P-channeltransistor.

Accordingly, since the basic circuit in FIG. 13A can be formed by usingonly P-channel transistors, a step of forming N-channel transistors isnot necessary. Thus, in the basic circuit in FIG. 13A, a manufacturingprocess can be simplified, so that manufacturing cost can be reduced anda yield can be improved.

In addition, the power supply potential VDD is supplied to the wiring1305 and the power supply potential VSS is supplied to the wiring 1306.

In addition, a signal is supplied to each of the wiring 1307 and thewiring 1308. Note that the signal supplied to each of the wiring 1307and the wiring 1308 is a binary digital signal.

Next, operations of the basic circuit shown in FIG. 13A are describedwith reference to FIG. 13B.

FIG. 13B is an example of a timing chart of the basic circuit shown inFIG. 13A. The timing chart in FIG. 13B shows a potential of the wiring1307, a potential of the wiring 1308, a potential of the node N131, apotential of the wiring 1309, and on/off of the transistor 1304.

The timing chart in FIG. 13B is described by dividing the whole periodinto periods T1 to T4. In addition, FIGS. 14A to 15B show operations ofthe basic circuit in FIG. 13A in the periods T1 to T4, respectively.

First, the operation in the period T1 is described with reference toFIG. 14A. In the period T1, an H-level signal is supplied to the wiring1307 and an if-level signal is supplied to the wiring 1308. Accordingly,the transistor 1302 is turned off and the transistor 1303 is off.

In addition, since the transistor 1301 is diode-connected, the potentialof the node N131 starts to decrease. This decrease in the potential ofthe node N131 continues until the transistor 1301 is turned off. Thetransistor 1301 is turned off when the potential of the node N131becomes the sum of the power supply potential VSS and the absolute valueof a threshold voltage Vth1301 of the transistor 1301 (VSS+|Vth1301|).Therefore, the potential of the node N131 becomes VSS+|Vth1301|.

Accordingly, the transistor 1304 is turned on and the potential of thewiring 1309 becomes equal to the power supply potential VDD.

Next, the operation in the period T2 is described with reference to FIG.14B. In the period T2, an L-level signal is supplied to the wiring 1307and an H-level signal is supplied to the wiring 1308. Accordingly, thetransistor 1302 is turned on and the transistor 1303 is off.

In addition, the potential of the node N131 is determined by theoperating point of the transistor 1301 and the transistor 1302. Notethat when a ratio (W/L) of the transistor 1302 (W means channel width ofa channel region and L means channel length of the channel region) isset sufficiently higher than a ratio (W/L) of the transistor 1301, thepotential of the node N131 becomes slightly lower than the power supplypotential VDD.

Accordingly, the transistor 1304 is turned off and the wiring 1309becomes a floating state. The potential of the wiring 1309 remains equalto the power supply potential VDD because the wiring 1309 is kept at thepotential in the period T1.

Next, the operation in the period T3 is described with reference to FIG.15A. In the period T3, an H-level signal is supplied to the wiring 1307and an L-level signal is supplied to the wiring 1308. Accordingly, thetransistor 1302 is turned off and the transistor 1303 is on.

In addition, the potential of the node N131 is determined by theoperating point of the transistor 1301 and the transistor 1303. Notethat when a ratio (W/L) of the transistor 1303 is set sufficientlyhigher than a ratio (W/L) of the transistor 1301, the potential of thenode N131 becomes slightly lower than the power supply potential VDD.

Accordingly, the transistor 1304 is turned off and the wiring 1309becomes a floating state. The potential of the wiring 1309 remains equalto the power supply potential VDD because the wiring 1309 is kept at thepotential in the periods T1 and T2.

Next, the operation in the period T4 is described with reference to FIG.15B. In the period T4, an L-level signal is supplied to the wiring 1307and an L-level signal is supplied to the wiring 1308. Accordingly, thetransistor 1302 is turned on and the transistor 1304 is on.

In addition, since the potential of the node N131 is determined by theoperating point of the transistor 1301, the transistor 1302, and thetransistor 1303, the potential of the node N131 becomes slightly lowerthan the power supply potential VDD.

Accordingly, the transistor 1304 is turned off and the wiring 1309becomes a floating state. The potential of the wiring 1309 remains equalto the power supply potential VDD because the wiring 1309 is kept at thepotential in the periods T1 to T3.

By the above-described operations, the basic circuit in FIG. 13Asupplies the power supply potential VDD to the wiring 1309 in the periodT1, so that the potential of the wiring 1309 becomes equal to the powersupply potential VDD. In the periods T2 to T4, the basic circuit in FIG.13A makes the wiring 1309 into a floating state, so that the potentialof the wiring 1309 is kept equal to the power supply potential VDD.

In addition, the basic circuit in FIG. 13A does not include a transistorwhich is on in all of the periods T1 to T4. That is, the basic circuitin FIG. 13A does not include a transistor which is always or almostalways on. Accordingly, the basic circuit in FIG. 13A can suppresscharacteristic deterioration of a transistor and a threshold voltageshift due to the characteristic deterioration.

Note that the transistors 1301 to 1304 have functions which are similarto those of the transistors 101 to 104.

Note that the transistor 1301 may be any element as long as it has aresistance component. For example, as shown in FIG. 16A, a resistor 1601can be used instead of the transistor 1301. By using the resistor 1601,the potential of the node N131 can be set equal to the power supplypotential VSS in the period T1. In addition, a timing chart in FIG. 16Ais shown in FIG. 16B.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 2

In this embodiment mode, a basic principle of the invention which isdifferent from that of Embodiment Mode 1 is described with reference toFIG. 5A.

FIG. 5A shows a basic circuit which is based on the basic principle ofthe invention. The basic circuit in FIG. 5A includes a transistor 501, atransistor 502, a transistor 503, a transistor 504, a transistor 505, atransistor 506, and a transistor 507.

Connection relations of the basic circuit in FIG. 5A are described. Agate of the transistor 501 is connected to a wiring 508, a firstterminal of the transistor 501 is connected to the wiring 508, and asecond terminal of the transistor 501 is connected to a gate of thetransistor 504. A gate of the transistor 502 is connected to a wiring510, a first terminal of the transistor 502 is connected to a wiring509, and a second terminal of the transistor 502 is connected to thegate of the transistor 504. A gate of the transistor 503 is connected toa wiring 511, a first terminal of the transistor 503 is connected to thewiring 509, and a second terminal of the transistor 503 is connected tothe gate of the transistor 504. Note that a node of the second terminalof the transistor 501, the second terminal of the transistor 502, thesecond terminal of the transistor 503, and the gate of the transistor504 is denoted by N51. A first terminal of the transistor 504 isconnected to the wiring 508, and a second terminal of the transistor 504is connected to a gate of the transistor 507. A gate of the transistor505 is connected to the wiring 510, a first terminal of the transistor505 is connected to the wiring 509, and a second terminal of thetransistor 505 is connected to the gate of the transistor 507. A gate ofthe transistor 506 is connected to the wiring 511, a first terminal ofthe transistor 506 is connected to the wiring 509, and a second terminalof the transistor 506 is connected to the gate of the transistor 507. Afirst terminal of the transistor 507 is connected to the wiring 509, anda second terminal of the transistor 507 is connected to a wiring 512.Note that a node of the second terminal of the transistor 504, thesecond terminal of the transistor 505, the second terminal of thetransistor 506, and the gate of the transistor 507 is denoted by N52.

In addition, each of the transistors 501 to 507 is an N-channeltransistor.

Accordingly, since the basic circuit in FIG. 5A can be formed by usingonly N-channel transistors, amorphous silicon can be used for asemiconductor layer of the basic circuit in FIG. 5A. Thus, amanufacturing process can be simplified, so that manufacturing cost canbe reduced and a yield can be improved. In addition, a semiconductordevice such as a large display panel can also be formed. Further, whenpolysilicon or single crystalline silicon is used for the semiconductorlayer of the basic circuit in FIG. 5A, the manufacturing process canalso be simplified.

In addition, the power supply potential VDD is supplied to the wiring508 and the power supply potential VSS is supplied to the wiring 509.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 508 and the wiring 509, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 510 and thewiring 511. Note that the signal supplied to each of the wiring 510 andthe wiring 511 is a binary digital signal. When the digital signal is anH-level signal, it has the same potential as the power supplypotential-VDD (hereinafter also referred to as a potential VDD or an Hlevel), and when the digital signal is an L-level signal, it has thesame potential as the power supply potential VSS (hereinafter alsoreferred to as a potential VSS or an L level). Note also that the powersupply potential VDD, the power supply potential VSS, or another powersupply potential may be supplied to each of the wiring 510 and thewiring 511. Alternatively, an analog signal may be supplied to each ofthe wiring 510 and the wiring 511.

Next, operations of the basic circuit shown in FIG. 5A are describedwith reference to FIG. 5B.

FIG. 5B is an example of a timing chart of the basic circuit shown inFIG. 5A. The timing chart in FIG. 5B shows a potential of the wiring510, a potential of the wiring 511, a potential of the node N51, apotential of the node N52, a potential of the wiring 512, and on/off ofthe transistor 507.

The timing chart in FIG. 5B is described by dividing the whole periodinto periods T1 to T4. In addition, FIGS. 6A to 7B show operations ofthe basic circuit in FIG. 5A in the periods T1 to T4, respectively.

First, the operation in the period T1 is described with reference toFIG. 6A. In the period T1, an L-level signal is supplied to the wiring510 and the transistors 502 and 505 are off. In addition, an L-levelsignal is supplied to the wiring 511 and the transistors 503 and 506 areoff.

In addition, since the transistor 501 is diode-connected, the potentialof the node N51 starts to rise. The transistor 501 is turned off whenthe potential of the node N51 becomes a value obtained by subtracting athreshold voltage Vth501 of the transistor 501 from the power supplypotential VDD (VDD−Vth501). Therefore, the node N51 becomes a floatingstate.

At this time, the transistor 504 is on and the potential of the node N52also rises. Accordingly, the potential of the node N51 which is in afloating state rises at the same time as the potential of the node N52by parasitic capacitance between the gate (the node N51) and the secondterminal (the node N52) of the transistor 504. This rise in thepotential of the node N51 continues until the rise in the potential ofthe node N52 is terminated, and the potential of the node N51 becomesequal to or higher than the sum of the power supply potential VDD and athreshold voltage Vth504 of the transistor 504 (VDD+Vth504). That is,the rise in the potential of the node N51 continues until the potentialof the node N52 becomes equal to the power supply potential VDD. Thepotential of the node N52 can be set equal to the power supply potentialVDD by performing a so-called bootstrap operation.

Accordingly, the transistor 507 is turned on and the potential of thewiring 509 becomes equal to the power supply potential VSS. Here, bysetting the potential of the node N52 to be equal to the power supplypotential VDD, a potential difference between the gate and a source ofthe transistor 507 can be increased. Therefore, the transistor 507 canbe easily turned on and the basic circuit can be operated under a widerange of operating conditions.

Next, the operation in the period T2 is described with reference to FIG.6B. In the period T2, an H-level signal is supplied to the wiring 510and the transistors 502 and 505 are on. In addition, an L-level signalis supplied to the wiring 511 and the transistors 503 and 506 are off.

In addition, the potential of the node N51 is determined by theoperating point of the transistor 501 and the transistor 502. Note thatwhen a ratio (W/L) of the transistor 502 is set sufficiently higher thana ratio (W/L) of the transistor 501, the potential of the node N51becomes slightly higher than the power supply potential VSS.

Accordingly, since the transistor 504 is turned off and the transistor505 is on, the potential of node N52 becomes equal to the power supplypotential VSS. Therefore, the transistor 507 is turned off and thewiring 512 becomes a floating state. The potential of the wiring 512remains equal to the power supply potential VSS because the wiring 512is kept at the potential in the period T1.

Next, the operation in the period T3 is described with reference to FIG.7A. In the period T3, an L-level signal is supplied to the wiring 510and the transistors 502 and 505 are off. In addition, an H-level signalis supplied to the wiring 511 and the transistors 503 and 506 are on.

In addition, the potential of the node N51 is determined by theoperating point of the transistor 501 and the transistor 503. Note thatwhen a ratio (W/L) of the transistor 503 is set sufficiently higher thana ratio (W/L) of the transistor 501, the potential of the node N51becomes slightly higher than the power supply potential VSS.

Accordingly, since the transistor 504 is turned off and the transistor506 is on, the potential of the node N52 becomes equal to the powersupply potential VSS. Therefore, the transistor 507 is turned off andthe wiring 512 becomes a floating state. The potential of the wiring 512remains equal to the power supply potential VSS because the wiring 512is kept at the potential in the periods T1 and T2.

Next, the operation in the period T4 is described with reference to FIG.7B. In the period T4, an H-level signal is supplied to the wiring 510and the transistors 502 and 505 are on. In addition, an H-level signalis supplied to the wiring 511 and the transistors 503 and 506 are on.

In addition, since the potential of the node N51 is determined by theoperating point of the transistor 501, the transistor 502, and thetransistor 503, the potential of the node N51 becomes slightly higherthan the power supply potential VSS.

Accordingly, since the transistor 504 is turned off and the transistors505 and 506 are on, the potential of the node N52 becomes equal to thepower supply potential VSS. Therefore, the transistor 507 is turned offand the wiring 512 becomes a floating state. The potential of the wiring512 remains equal to the power supply potential VSS because the wiring512 is kept at the potential in the periods T1 to T3.

By the above-described operations, the basic circuit in FIG. 5A suppliesthe power supply potential VSS to the wiring 512 in the period T1, sothat the potential of the wiring 512 becomes equal to the power supplypotential VSS. In the periods T2 to T4, the basic circuit in FIG. SAmakes the wiring 512 into a floating state, so that the potential of thewiring 512 is kept equal to the power supply potential VSS.

Note that the potential of the node N52 of the basic circuit in FIG. 5Acan be set equal to the power supply potential VDD in the period T1.Therefore, the basic circuit in FIG. 5A can be operated under a widerange of operating conditions.

In addition, the basic circuit in FIG. 5A does not include a transistorwhich is on in all of the periods T1 to T4. That is, the basic circuitin FIG. 5A does not include a transistor which is always or almostalways on. Accordingly, the basic circuit in FIG. 5A can suppresscharacteristic deterioration of a transistor and a threshold voltageshift due to the characteristic deterioration.

Further, the characteristics of a transistor which is formed ofamorphous silicon easily deteriorate. Therefore, when the transistorincluded in the basic circuit in FIG. 5A is formed using amorphoussilicon, not only can the advantages such as a reduction inmanufacturing cost and improvement in a yield be obtained, but also theproblem of the characteristic deterioration of the transistor can besolved.

Here, the functions of the transistors 501 to 507 are described. Thetransistor 501 has a function of a diode in which the first terminal andthe gate correspond to an input terminal and the second terminalcorresponds to an output terminal. The transistor 502 has a function ofa switch which selects whether to connect the wiring 509 and the nodeN51 in accordance with the potential of the wiring 510. The transistor503 has a function of a switch which selects whether to connect thewiring 509 and the node N51 in accordance with the potential of thewiring 511. The transistor 504 has a function of a switch which selectswhether to connect the wiring 508 and the node N52 in accordance withthe potential of the node N51. The transistor 505 has a function of aswitch which selects whether to connect the wiring 509 and the node N52in accordance with the potential of the wiring 510. The transistor 506has a function of a switch which selects whether to connect the wiring509 and the node N52 in accordance with the potential of the wiring 511.The transistor 507 has a function of a switch which selects whether toconnect the wiring 509 and the wiring 512 in accordance with thepotential of the node N52.

Note that a two-input NOR circuit in which the wirings 510 and 511correspond to an input terminal and the node N52 corresponds to anoutput terminal is constructed from the transistors 501 to 506.

Note that as shown in FIG. 8A, a capacitor 801 may be provided betweenthe gate (the node N51) and the second terminal (the node N52) of thetransistor 504. This is because the potential of the node N51 and thepotential of the node N52 are raised by the bootstrap operation, so thatthe basic circuit can easily perform the bootstrap operation by provingthe capacitor 801.

Note also that as shown in FIG. 8B, the transistor 503 is notnecessarily provided. This is because when an H-level signal is suppliedto the wiring 510, it is only necessary that the potential of the nodeN52 be decreased to turn off the transistor 507.

Next, the case is described in which the basic circuit shown in FIG. 5Ais constructed from P-channel transistors, with reference to FIG. 17A.

FIG. 17A shows a basic circuit which is based on the basic principle ofthe invention. The basic circuit in FIG. 17A includes a transistor 1701,a transistor 1702, a transistor 1703, a transistor 1704, a transistor1705, a transistor 1706, and a transistor 1707.

Connection relations of the basic circuit in FIG. 17A are described. Agate of the transistor 1701 is connected to a wiring 1709, a firstterminal of the transistor 1701 is connected to the wiring 1709, and asecond terminal of the transistor 1701 is connected to a gate of thetransistor 1704. A gate of the transistor 1702 is connected to a wiring1710, a first terminal of the transistor 1702 is connected to a wiring1708, and a second terminal of the transistor 1702 is connected to thegate of the transistor 1704. A gate of the transistor 1703 is connectedto a wiring 1711, a first terminal of the transistor 1703 is connectedto the wiring 1708, and a second terminal of the transistor 1703 isconnected to the gate of the transistor 1704. Note that a node of thesecond terminal of the transistor 1701, the second terminal of thetransistor 1702, the second terminal of the transistor 1703, and thegate of the transistor 1704 is denoted by N171. A first terminal of thetransistor 1704 is connected to the wiring 1709, and a second terminalof the transistor 1704 is connected to a gate of the transistor 1707. Agate of the transistor 1705 is connected to the wiring 1710, a firstterminal of the transistor 1705 is connected to the wiring 1708, and asecond terminal of the transistor 1705 is connected to the gate of thetransistor 1707. A gate of the transistor 1706 is connected to thewiring 1711, a first terminal of the transistor 1706 is connected to thewiring 1708, and a second terminal of the transistor 1706 is connectedto the gate of the transistor 1707. A first terminal of the transistor1707 is connected to the wiring 1708, and a second terminal of thetransistor 1707 is connected to a wiring 1712. Note that a node of thesecond terminal of the transistor 1704, the second terminal of thetransistor 1705, the second terminal of the transistor 1706, and thegate of the transistor 1707 is denoted by N172.

In addition, each of the transistors 1701 to 1707 is a P-channeltransistor.

Accordingly, since the basic circuit in FIG. 17A can be formed by usingonly P-channel transistors, a step of forming N-channel transistors isnot necessary. Thus, in the basic circuit in FIG. 17A, a manufacturingprocess can be simplified, so that manufacturing cost can be reduced anda yield can be improved.

In addition, the power supply potential VDD is supplied to the wiring1708 and the power supply potential VSS is supplied to the wiring 1709.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 1708 and the wiring 1709, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 1710 and thewiring 1711. Note that the signal supplied to each of the wiring 1710and the wiring 1711 is a binary digital signal. Note also that the powersupply potential VDD, the power supply potential VSS, or another powersupply potential may be supplied to each of the wiring 1710 and thewiring 1711. Alternatively, an analog signal may be supplied to each ofthe wiring 1710 and the wiring 1711.

Next, operations of the basic circuit shown in FIG. 17A are describedwith reference to FIG. 17B.

FIG. 17B is an example of a timing chart of the basic circuit shown inFIG. 17A. The timing chart in FIG. 17B shows a potential of the wiring1710, a potential of the wiring 1711, a potential of the node N171, apotential of the node N172, a potential of the wiring 1712, and on/offof the transistor 1707.

The timing chart in FIG. 1711 is described by dividing the whole periodinto periods T1 to T4. In addition, FIGS. 18A to 19B show operations ofthe basic circuit in FIG. 17A in the periods T1 to T4, respectively.

First, the operation in the period T1 is described with reference to HU.18A. In the period T1, an H-level signal is supplied to the wiring 1710and the transistors 1702 and 1705 are off. In addition, an H-levelsignal is supplied to the wiring 1711 and the transistors 1703 and 1706are off.

In addition, since the transistor 1701 is diode-connected, the potentialof the node N171 starts to decrease. The transistor 1701 is turned offwhen the potential of the node N171 becomes the sum of the power supplypotential VSS and the absolute value of a threshold voltage Vth1701 ofthe transistor 1701 (VSS+|Vth1701|). Therefore, the node N171 becomes afloating state.

At this time, the transistor 1704 is on and the potential of the nodeN172 also decreases. Accordingly, the potential of the node N171 whichis in a floating state decreases at the same time as the potential ofthe node N172 by parasitic capacitance between the gate (the node N171)and the second terminal (the node N172) of the transistor 1704. Thisdecrease in the potential of the node N171 continues until the decreasein the potential of the node N172 is terminated, and the potential ofthe node N171 becomes equal to or lower than a value obtained bysubtracting the absolute value of a threshold voltage Vth1704 of thetransistor 1704 from the power supply potential VSS (VSS−|Vth1704|).That is, the decrease in the potential of the node N171 continues untilthe potential of the node N172 becomes equal to the power supplypotential VSS. The potential of the node N172 can be set equal to thepower supply potential VSS by performing a so-called bootstrapoperation.

Accordingly, the transistor 1707 is turned on and the potential of thewiring 1712 becomes equal to the power supply potential VSS. Here, bysetting the potential of the node N172 to be equal to the power supplypotential VSS, a potential difference between the gate and a source ofthe transistor 1707 can be increased. Therefore, the transistor 1707 canbe easily turned on and the basic circuit can be operated under a widerange of operating conditions.

Next, the operation in the period T2 is described with reference to FIG.18B. In the period T2, an L-level signal is supplied to the wiring 1710and the transistors 1702 and 1705 are on. In addition, an H-level signalis supplied to the wiring 1711 and the transistors 1703 and 1706 areoff.

In addition, the potential of the node N171 is determined by theoperating point of the transistor 1701 and the transistor 1702. Notethat when a ratio (W/L) of the transistor 1702 is set sufficientlyhigher than a ratio (W/L) of the transistor 1701, the potential of thenode N171 becomes slightly lower than the power supply potential VDD.

Accordingly, since the transistor 1704 is turned off and the transistor1705 is on, the potential of the node N172 becomes equal to the powersupply potential VDD. Therefore, the transistor 1707 is turned off andthe wiring 1712 becomes a floating state. The potential of the wiring1712 remains equal to the power supply potential VDD because the wiring1712 is kept at the potential in the period T1.

Next, the operation in the period T3 is described with reference to FIG.19A. In the period T3, an H-level signal is supplied to the wiring 1710and the transistors 1702 and 1705 are off. In addition, an L-levelsignal is supplied to the wiring 1711 and the transistors 1703 and 1706are on.

In addition, the potential of the node N171 is determined by theoperating point of the transistor 1701 and the transistor 1703. Notethat when a ratio (W/L) of the transistor 1703 is set sufficientlyhigher than a ratio (W/L) of the transistor 1701, the potential of thenode N171 becomes slightly lower than the power supply potential VDD.

Accordingly, since the transistor 1704 is turned off and the transistor1706 is on, the potential of the node N172 becomes equal to the powersupply potential VDD. Therefore, the transistor 1707 is turned off andthe wiring 1712 becomes a floating state. The potential of the wiring1712 remains equal to the power supply potential VDD because the wiring1712 is kept at the potential in the periods T1 and T2.

Next, the operation in the period T4 is described with reference to FIT.19B. In the period T4, an L-level signal is supplied to the wiring 1710and the transistors 1702 and 1705 are on. In addition, an L-level signalis supplied to the wiring 1711 and the transistors 1703 and 1706 are on.

In addition, since the potential of the node N171 is determined by theoperating point of the transistor 1701, the transistor 1702, and thetransistor 1703, the potential of the node N171 becomes slightly lowerthan the power supply potential VDD.

Accordingly, since the transistor 1704 is turned off and the transistors1705 and 1706 are on, the potential of the node N172 becomes equal tothe power supply potential VDD. Therefore, the transistor 1707 is turnedoff and the wiring 1712 becomes a floating state. The potential of thewiring 1712 remains equal to the power supply potential VDD because thewiring 1712 is kept at the potential in the periods T1 to T3.

By the above-described operations, the basic circuit in FIG. 17Asupplies the power supply potential VDD to the wiring 1712 in the periodT1, so that the potential of the wiring 1712 becomes equal to the powersupply potential VDD. In the periods T2 to T4, the basic circuit in FIG.17A makes the wiring 1712 into a floating state, so that the potentialof the wiring 1712 is kept equal to the power supply potential VDD.

Note that the potential of the node N172 of the basic circuit in FIG.17A can be set equal to the power supply potential VSS in the period T1.Therefore, the basic circuit in FIG. 17A can be operated under a widerange of operating conditions.

In addition, the basic circuit in FIG. 17A does not include a transistorwhich is on in all of the periods T1 to T4. That is, the basic circuitin FIG. 17A does not include a transistor which is always or almostalways on. Accordingly, the basic circuit in FIG. 17A can suppresscharacteristic deterioration of a transistor and a threshold voltageshift due to the characteristic deterioration.

Note that the transistors 1701 to 1707 have functions which are similarto those of the transistors 501 to 507.

Note that a two-input NAND circuit in which the wirings 1710 and 1711correspond to an input terminal and the node N172 corresponds to anoutput terminal is constructed from the transistors 1701 to 1706.

Note that as shown in FIG. 20A, a capacitor 2001 may be provided betweenthe gate (the node N171) and the second terminal (the node N172) of thetransistor 1704. This is because the potential of the node N171 and thepotential of the node N172 are raised by the bootstrap operation, sothat the basic circuit can easily perform the bootstrap operation byproving the capacitor 2001.

Note also that as shown in FIG. 20B, the transistor 1703 is notnecessarily provided. This is because when an L-level signal is suppliedto the wiring 1710, it is only necessary that the potential of the nodeN172 be raised to turn off the transistor 1707.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 3

In this embodiment mode, a basic principle of the invention which isdifferent from those of Embodiment Modes 1 and 2 is described withreference to FIG. 9A.

FIG. 9A shows a basic circuit which is based on the basic principle ofthe invention. The basic circuit in FIG. 9A includes a transistor 901, atransistor 902, a transistor 903, and a transistor 904.

Connection relations of the basic circuit in FIG. 9A are described. Agate of the transistor 901 is connected to a gate of the transistor 904,a first terminal of the transistor 901 is connected to a wiring 906, anda second terminal of the transistor 901 is connected to the gate of thetransistor 904. A gate of the transistor 902 is connected to a wiring907, a first terminal of the transistor 902 is connected to a wiring905, and a second terminal of the transistor 902 is connected to thegate of the transistor 904. A gate of the transistor 903 is connected toa wiring 908, a first terminal of the transistor 903 is connected to thewiring 906, and a second terminal of the transistor 903 is connected tothe gate of the transistor 904. A first terminal of the transistor 904is connected to the wiring 906, and a second terminal of the transistor904 is connected to a wiring 909. Note that a node of the secondterminal of the transistor 901, the gate of the transistor 901, thesecond terminal of the transistor 902, the second terminal of thetransistor 903, and the gate of the transistor 904 is denoted by N91.

In addition, each of the transistors 901 to 904 is an N-channeltransistor.

Accordingly, since the basic circuit in FIG. 9A can be formed by usingonly N-channel transistors, amorphous silicon can be used for asemiconductor layer of the basic circuit in FIG. 9A. Thus, amanufacturing process can be simplified, so that manufacturing cost canbe reduced and a yield can be improved. In addition, a semiconductordevice such as a large display panel can also be formed. Further, whenpolysilicon or single crystalline silicon is used for the semiconductorlayer of the basic circuit in FIG. 9A, the manufacturing process canalso be simplified.

In addition, the power supply potential VDD is supplied to the wiring905 and the power supply potential VSS is supplied to the wiring 906.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 905 and the wiring 906, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 907 and thewiring 908. Note that the signal supplied to each of the wiring 907 andthe wiring 908 is a binary digital signal. Note also that the powersupply potential VDD, the power supply potential VSS, or another powersupply potential may be supplied to each of the wiring 907 and thewiring 908. Alternatively, an analog signal may be supplied to each ofthe wiring 907 and the wiring 908.

Next, operations of the basic circuit shown in FIG. 9A are describedwith reference to FIG. 9B.

FIG. 9B is an example of a timing chart of the basic circuit shown inFIG. 9A. The timing chart in FIG. 9B shows a potential of the wiring907, a potential of the wiring 908, a potential of the node N91, apotential of the wiring 909, and on/off of the transistor 904.

The timing chart in FIG. 9B is described by dividing the whole periodinto periods T1 to T4. In addition, FIGS. 10A to 11B show operations ofthe basic circuit in FIG. 9A in the periods T1 to T4, respectively.

First, the operation in the period T1 is described with reference toFIG. 10A. In the period T1, an L-level signal is supplied to the wiring907 and an L-level signal is supplied to the wiring 908. Accordingly,the transistor 902 is turned off and the transistor 903 is off.

In addition, since the transistor 901 is diode-connected, the potentialof the node N91 starts to decrease. This decrease in the potential ofthe node N91 continues until the transistor 901 is turned off. Thetransistor 901 is turned off when the potential of the node N91 becomesthe sum of the power supply potential VSS and the absolute value of athreshold voltage Vth901 of the transistor 901 (VSS+|Vth901|).Therefore, the potential of the node N91 becomes VSS+|Vth901|.

Accordingly, the transistor 904 is turned off, and the potential of thewiring 909 remains equal to the power supply potential VSS because thewiring 909 is kept at a potential in the period T2. Note that theoperation in the period T2 is described next.

Next, the operation in the period T2 is described with reference to FIG.10B. In the period T2, an H-level signal is supplied to the wiring 907and an L-level signal is supplied to the wiring 908. Accordingly, thetransistor 902 is turned on and the transistor 903 is off.

In addition, the potential of the node N91 is determined by theoperating point of the transistor 901 and the transistor 902. Note thatwhen a ratio (W/L) of the transistor 902 is set sufficiently higher thana ratio (W/L) of the transistor 901, the potential of the node N91becomes slightly lower than the power supply potential VDD.

Accordingly, the transistor 904 is turned on and the potential of thewiring 909 becomes equal to the power supply potential VSS.

Next, the operation in the period T3 is described with reference to FIG.11A. In the period T3, an L-level signal is supplied to the wiring 907and an H-level signal is supplied to the wiring 908. Accordingly, thetransistor 902 is turned off and the transistor 903 is on.

Accordingly, the potential of the node N91 becomes equal to the powersupply potential VSS because the transistor 904 is off.

Accordingly, the transistor 904 is turned off and the wiring 909 becomesa floating state. The potential of the wiring 909 remains equal to thepower supply potential VSS because the wiring 909 is kept at thepotential in the periods T1 and T2.

Next, the operation in the period T4 is described with reference to FIG.11B. In the period T4, an H-level signal is supplied to the wiring 907and an H-level signal is supplied to the wiring 908. Accordingly, thetransistor 902 is turned on and the transistor 904 is on.

In addition, since the potential of the node N91 is determined by theoperating point of the transistor 901, the transistor 902, and thetransistor 903, the potential of the node N91 becomes slightly higherthan the power supply potential VSS.

Accordingly, the transistor 904 is turned off and the wiring 909 becomesa floating state. The potential of the wiring 909 remains equal to thepower supply potential VSS because the wiring 909 is kept at thepotential in the periods T1 to T3.

By the above-described operations, the basic circuit in FIG. 9A suppliesthe power supply potential VSS to the wiring 909 in the period T2, sothat the potential of the wiring 909 becomes equal to the power supplypotential VSS. In the periods T1, T3, and T4, the basic circuit in FIG.9A makes the wiring 909 into a floating state, so that the potential ofthe wiring 909 is kept equal to the power supply potential VSS.

In addition, the basic circuit in FIG. 9A does not include a transistorwhich is on in all of the periods T1 to T4. That is, the basic circuitin FIG. 9A does not include a transistor which is always or almostalways on. Accordingly, the basic circuit in FIG. 9A can suppresscharacteristic deterioration of a transistor and a threshold voltageshift due to the characteristic deterioration.

Further, the characteristics of a transistor which is formed ofamorphous silicon easily deteriorate. Therefore, when the transistorincluded in the basic circuit in FIG. 9A is formed using amorphoussilicon, not only can the advantages such as a reduction inmanufacturing cost and improvement in a yield be obtained, but also theproblem of the characteristic deterioration of the transistor can besolved.

Here, the functions of the transistors 901 to 904 are described. Thetransistor 901 has a function of a diode in which the second terminaland the gate correspond to an input terminal and the first terminalcorresponds to an output terminal. The transistor 902 has a function ofa switch which selects whether to connect the wiring 905 and the nodeN91 in accordance with the potential of the wiring 907. The transistor903 has a function of a switch which selects whether to connect thewiring 906 and the node N91 in accordance with the potential of thewiring 908. The transistor 904 has a function of a switch which selectswhether to connect the wiring 906 and the wiring 909 in accordance withthe potential of the node N91.

Note that a two-input logic circuit in which the wirings 907 and 908correspond to an input terminal and the node N91 corresponds to anoutput terminal is constructed from the transistors 901 to 904.

Note that the transistor 901 may be any element as long as it has aresistance component. For example, as shown in FIG. 12A, a resistor 1201can be used instead of the transistor 901. In addition, a timing chartin FIG. 12A is shown in FIG. 12B.

Next, the case is described in which the basic circuit shown in FIG. 9Ais constructed from P-channel transistors, with reference to FIG. 21A.

FIG. 21A shows a basic circuit which is based on the basic principle ofthe invention. The basic circuit in FIG. 21A includes a transistor 2101,a transistor 2102, a transistor 2103, and a transistor 2104.

Connection relations of the basic circuit in FIG. 21A are described. Agate of the transistor 2101 is connected to a gate of the transistor2104, a first terminal of the transistor 2101 is connected to a wiring2105, and a second terminal of the transistor 2101 is connected to thegate of the transistor 2104. A gate of the transistor 2102 is connectedto a wiring 2107, a first terminal of the transistor 2102 is connectedto a wiring 2106, and a second terminal of the transistor 2102 isconnected to the gate of the transistor 2104. A gate of the transistor2103 is connected to a wiring 2108, a first terminal of the transistor2103 is connected to the wiring 2105, and a second terminal of thetransistor 2103 is connected to the gate of the transistor 2104. A firstterminal of the transistor 2104 is connected to the wiring 2105, and asecond terminal of the transistor 2104 is connected to a wiring 2109.Note that a node of the gate of the transistor 2101, the second terminalof the transistor 2101, the second terminal of the transistor 2102, thesecond terminal of the transistor 2103, and the gate of the transistor2104 is denoted by N211.

In addition, each of the transistors 2101 to 2104 is a P-channeltransistor.

Accordingly since the basic circuit in FIG. 21A can be formed by usingonly P-channel transistors, a step of forming N-channel transistors isnot necessary. Thus, in the basic circuit in FIG. 21A, a manufacturingprocess can be simplified, so that manufacturing cost can be reduced anda yield can be improved.

In addition, the power supply potential VDD is supplied to the wiring2105 and the power supply potential VSS is supplied to the wiring 2106.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 2105 and the wiring 2106, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 2107 and thewiring 2108. Note that the signal supplied to each of the wiring 2107and the wiring 2108 is a binary digital signal. Note also that the powersupply potential VDD, the power supply potential VSS, or another powersupply potential may be supplied to each of the wiring 2107 and thewiring 2108. Alternatively, an analog signal may be supplied to each ofthe wiring 2107 and the wiring 2108.

Next, operations of the basic circuit shown in FIG. 21A are describedwith reference to FIG. 21B.

FIG. 21B is an example of a timing chart of the basic circuit shown inFIG. 21A. The timing chart in FIG. 21B shows a potential of the wiring2107, a potential of the wiring 2108, a potential of the node N211, apotential of the wiring 2109, and on/off of the transistor 2104.

The timing chart in FIG. 21B is described by dividing the whole periodinto periods T1 to T4. In addition, FIGS. 22A to 23B show operations ofthe basic circuit in FIG. 21A in the periods T1 to T4, respectively.

First, the operation in the period T1 is described with reference toFIG. 22A. In the period T1, an H-level signal is supplied to the wiring2107 and an H-level signal is supplied to the wiring 2108. Accordingly,the transistor 2102 is turned off and the transistor 2103 is off.

In addition, since the transistor 2101 is diode-connected, the potentialof the node N211 starts to rise. This rise in the potential of the nodeN211 continues until the transistor 2101 is turned off. The transistor2101 is turned off when the potential of the node N211 becomes a valueobtained by subtracting the absolute value of a threshold voltageVth2101 of the transistor 2101 from the power supply potential VDD(VDD−|Vth2101|). Therefore, the potential of the node N211 becomesVDD−|Vth2101|.

Accordingly, the transistor 2104 is turned off, and the potential of thewiring 2109 remains slightly lower than the power supply potential VDDbecause the wiring 2109 is kept at a potential in the period T2. Notethat the operation in the period T2 is described next.

Next, the operation in the period T2 is described with reference to FIG.22B. In the period T2, an L-level signal is supplied to the wiring 2107and an H-level signal is supplied to the wiring 2108. Accordingly, thetransistor 2102 is turned on and the transistor 2103 is off.

In addition, the potential of the node N211 is determined by theoperating point of the transistor 2101 and the transistor 2102. Notethat when a ratio (W/L) of the transistor 2102 is set sufficientlyhigher than a ratio (W/L) of the transistor 2101, the potential of thenode N211 becomes slightly higher than the power supply potential VSS.

Accordingly, the transistor 2104 is turned on and the potential of thewiring 2109 becomes equal to the power supply potential VDD.

Next, the operation in the period T3 is described with reference to FIG.23A. In the period T3, an H-level signal is supplied to the wiring 2107and an L-level signal is supplied to the wiring 2108. Accordingly, thetransistor 2102 is turned off and the transistor 2103 is on.

Accordingly, the potential of the node N211 becomes equal to the powersupply potential VDD because the transistor 2102 is off.

Accordingly, the transistor 2104 is turned off and the wiring 2109becomes a floating state. The potential of the wiring 2109 remains equalto the power supply potential VSS because the wiring 2109 is kept at thepotential in the periods T1 and T2.

Next, the operation in the period T4 is described with reference to FIG.23B. In the period T4, an L-level signal is supplied to the wiring 2107and an L-level signal is supplied to the wiring 2108. Accordingly, thetransistor 2102 is turned on and the transistor 2104 is on.

In addition, since the potential of the node N211 is determined by theoperating point of the transistor 2101, the transistor 2102, and thetransistor 2103, the potential of the node N211 becomes slightly lowerthan the power supply potential VDD.

Accordingly, the transistor 2104 is turned off and the wiring 2109becomes a floating state. The potential of the wiring 2109 remains equalto the power supply potential VSS because the wiring 2109 is kept at thepotential in the periods T1 to T3.

By the above-described operations, the basic circuit in FIG. 21Asupplies the power supply potential VDD to the wiring 2109 in the periodT2, so that the potential of the wiring 2109 becomes equal to the powersupply potential VDD. In the periods T1, T3, and T4, the basic circuitin FIG. 21A makes the wiring 2109 into a floating state, so that thepotential of the wiring 2109 is kept equal to the power supply potentialVDD.

In addition, the basic circuit in FIG. 21A does not include a transistorwhich is on in all of the periods T1 to T4. That is, the basic circuitin FIG. 21A does not include a transistor which is always or almostalways on. Accordingly, the basic circuit in FIG. 21A can suppresscharacteristic deterioration of a transistor and a threshold voltageshift due to the characteristic deterioration.

Note that the transistors 2101 to 2104 have functions which are similarto those of the transistors 901 to 904.

Note that a two-input logic circuit in which the wirings 2107 and 2108correspond to an input terminal and the node N211 corresponds to anoutput terminal is constructed from the transistors 2101 to 2104.

Note that the transistor 2101 may be any element as long as it has aresistance component. For example, as shown in FIG. 24A, a resistor 2401can be used instead of the transistor 2101. In addition, a timing chartin FIG. 24A is shown in FIG. 24B.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 4

In this embodiment mode, a basic principle of the invention which isdifferent from those of Embodiment Modes 1 to 3 is described withreference to FIG. 25A.

FIG. 25A shows a basic circuit based on the basic principle of theinvention. The basic circuit in FIG. 25A includes a circuit 2501 and acircuit 2502.

Note that as the circuit 2501 and the circuit 2502, the basic circuitsshown in FIGS. 1A, 4A, 5A, 8A, 8B, 9A, and 12A can be used.

Therefore, a wiring 2503 and a wiring 2504 correspond to the wiring 107in FIG. 1A, the wiring 107 in FIG. 4A, the wiring 510 in FIG. 5A, thewiring 510 in FIG. 8A, the wiring 510 in FIG. 8B, the wiring 907 in FIG.9A, and the wiring 907 in FIG. 12A.

In addition, a wiring 2505 corresponds to the wiring 108 in FIG. 1A, thewiring 108 in FIG. 4A, the wiring 511 in FIG. 5A, the wiring 511 in FIG.8A, the wiring 511 in FIG. 8B, the wiring 908 in FIG. 9A, and the wiring908 in FIG. 12A.

In addition, a wiring 2506 corresponds to the wiring 109 in FIG. 1A, thewiring 109 in FIG. 4A, the wiring 512 in FIG. 5A, the wiring 512 in FIG.8A, the wiring 512 in FIG. 8B, the wiring 909 in FIG. 9A, and the wiring909 in FIG. 12A.

Accordingly, since the basic circuit in FIG. 25A can be formed by usingonly N-channel transistors, amorphous silicon can be used for asemiconductor layer of the basic circuit in FIG. 25A. Thus, amanufacturing process can be simplified, so that manufacturing cost canbe reduced and a yield can be improved. In addition, a semiconductordevice such as a large display panel can also be formed. Further, whenpolysilicon or single crystalline silicon is used for the semiconductorlayer of the basic circuit in FIG. 25A, the manufacturing process canalso be simplified.

In addition, a wiring to which a power supply potential is supplied isomitted.

In addition, a signal is supplied to each of the wiring 2503, the wiring2504, and the wiring 2505. Note that the signal supplied to each of thewiring 2503, the wiring 2504, and the wiring 2505 is a binary digitalsignal.

Note also that the power supply potential VDD, the power supplypotential VSS, or another power supply potential may be supplied to eachof the wiring 2503, the wiring 2504, and the wiring 2505. Alternatively,an analog signal may be supplied to each of the wiring 2503, the wiring2504, and the wiring 2505.

Next, operations of the basic circuit shown in FIG. 25A are describedwith reference to FIG. 25B. Note that FIG. 25B shows the case in whichthe basic circuits shown in FIGS. 1A, 4A, 5A, and 8A are used as thecircuit 2501 and the circuit 2502.

FIG. 25B is an example of a timing chart of the basic circuit shown inFIG. 25A. The timing chart in FIG. 25B shows a potential of the wiring2503, a potential of the wiring 2504, a potential of the wiring 2505,whether the output of the circuit 2501 is in a floating state (describedas OFF) or at the power supply potential VSS (described as ON), whetherthe output of the circuit 2502 is in a floating state (described as OFF)or at the power supply potential VSS (described as ON), and a potentialof the wiring 2506.

The timing chart in FIG. 25B is described by dividing the whole periodinto periods T1 to T8.

First, an operation in the period T1 is described. In the period T1, anL-level signal is supplied to the wiring 2505, an L-level signal issupplied to the wiring 2503, and an L-level signal is supplied to thewiring 2504. Each of the circuit 2501 and the circuit 2502 supplies thepower supply potential VSS to the wiring 2506. Therefore, the potentialof the wiring 2506 becomes equal to the power supply potential VSS.

Next, an operation in the period T2 is described. In the period T2, anL-level signal is supplied to the wiring 2505, an H-level signal issupplied to the wiring 2503, and an L-level signal is supplied to thewiring 2504. The circuit 2501 supplies no potential to the wiring 2506and the circuit 2502 supplies the power supply potential VSS to thewiring 2506. Therefore, the potential of the wiring 2506 becomes equalto the power supply potential VSS.

Next, an operation in the period T3 is described. In the period T3, anL-level signal is supplied to the wiring 2505, an L-level signal issupplied to the wiring 2503, and an H-level signal is supplied to thewiring 2504. The circuit 2501 supplies the power supply potential VSS tothe wiring 2506 and the circuit 2502 supplies no potential to the wiring2506. Therefore, the potential of the wiring 2506 becomes equal to thepower supply potential VSS.

Next, an operation in the period T4 is described. In the period T4, anL-level signal is supplied to the wiring 2505, an H-level signal issupplied to the wiring 2503, and an H-level signal is supplied to thewiring 2504. Each of the circuit 2501 and the circuit 2502 supplies nopotential to the wiring 2506. Therefore, the potential of the wiring2506 remains equal to the power supply potential VSS because the wiring2506 is kept at the potential in the period T3.

Next, an operation in the period T5 is described. In the period T5, anH-level signal is supplied to the wiring 2505, an L-level signal issupplied to the wiring 2503, and an L-level signal is supplied to thewiring 2504. Each of the circuit 2501 and the circuit 2502 supplies nopotential to the wiring 2506. Therefore, the potential of the wiring2506 remains equal to the power supply potential VSS because the wiring2506 is kept at the potential in the period T3.

Next, an operation in the period T6 is described. In the period T6, anH-level signal is supplied to the wiring 2505, an H-level signal issupplied to the wiring 2503, and an L-level signal is supplied to thewiring 2504. Each of the circuit 2501 and the circuit 2502 supplies nopotential to the wiring 2506. Therefore, the potential of the wiring2506 remains equal to the power supply potential VSS because the wiring2506 is kept at the potential in the period T3.

Next, an operation in the period T7 is described. In the period T7, anH-level signal is supplied to the wiring 2505, an L-level signal issupplied to the wiring 2503, and an H-level signal is supplied to thewiring 2504. Each of the circuit 2501 and the circuit 2502 supplies nopotential to the wiring 2506. Therefore, the potential of the wiring2506 remains equal to the power supply potential VSS because the wiring2506 is kept at the potential in the period T3.

Next, an operation in the period T8 is described. In the period T8, anH-level signal is supplied to the wiring 2505, an H-level signal issupplied to the wiring 2503, and an H-level signal is supplied to thewiring 2504. Each of the circuit 2501 and the circuit 2502 supplies nopotential to the wiring 2506. Therefore, the potential of the wiring2506 remains equal to the power supply potential VSS because the wiring2506 is kept at the potential in the period T3.

By the above-described operations, each of the circuit 2501 and thecircuit 2502 supplies the power supply potential VSS to the wiring 2506in the period T1, so that the potential of the wiring 2506 becomes equalto the power supply potential VSS. In the period T2, the circuit 2502supplies the power supply potential VSS to the wiring 2506, so that thepotential of the wiring 2506 becomes equal to the power supply potentialVSS. In the period T3, the circuit 2501 supplies the power supplypotential VSS to the wiring 2506, so that the potential of the wiring2506 becomes equal to the power supply potential VSS. In the periods T4to T8, the wiring 2506 is made into a floating state, so that thepotential of the wiring 2506 is kept equal to the power supply potentialVSS.

In addition, the basic circuit in FIG. 25A does not include a transistorwhich is on in all of the periods T1 to T8. That is, the basic circuitin FIG. 25A does not include a transistor which is always or almostalways on. Accordingly, the basic circuit in FIG. 25A can suppresscharacteristic deterioration of a transistor and a threshold voltageshift due to the characteristic deterioration.

Further, the characteristics of a transistor which is formed ofamorphous silicon easily deteriorate. Therefore, when the transistorincluded in the basic circuit in FIG. 25A is formed using amorphoussilicon, not only can the advantages such as a reduction inmanufacturing cost and improvement in a yield be obtained, but also theproblem of the characteristic deterioration of the transistor can besolved.

Next, the case is described in which the basic circuit shown in FIG. 25Ais constructed from P-channel transistors, with reference to FIG. 26A.

FIG. 26A shows a basic circuit which is based on the basic principle ofthe invention. The basic circuit in FIG. 26A includes a circuit 2601 anda circuit 2602.

Note that as the circuit 2601 and the circuit 2602, the basic circuitsshown in FIGS. 13A, 16A, 17A, 20A, 20B, 21A, and 24A can be used.

Therefore, a wiring 2603 and a wiring 2604 correspond to the wiring 1307in FIG. 13A, the wiring 1307 in FIG. 16A, the wiring 1710 in FIG. 17A,the wiring 1710 in FIG. 20A, the wiring 1710 in FIG. 20B, the wiring2108 in FIG. 21A, and the wiring 2108 in FIG. 24A.

In addition, a wiring 2605 corresponds to the wiring 1308 in FIG. 13A,the wiring 1308 in FIG. 16A, the wiring 1711 in FIG. 17A, the wiring1711 in FIG. 20A, the wiring 1711 in FIG. 20B, the wiring 2107 in FIG.21A, and the wiring 2107 in FIG. 24A.

In addition, a wiring 2606 corresponds to the wiring 1309 in FIG. 13A,the wiring 1309 in FIG. 16A, the wiring 1712 in FIG. 17A, the wiring1712 in FIG. 20A, the wiring 1712 in FIG. 20B, the wiring 2109 in FIG.21A, and the wiring 2109 in FIG. 24A.

Accordingly, since the basic circuit in FIG. 26A can be formed by usingonly P-channel transistors, a step of forming N-channel transistors isnot necessary. Thus, in the basic circuit in FIG. 26A, a manufacturingprocess can be simplified, so that manufacturing cost can be reduced anda yield can be improved.

In addition, a wiring to which a power supply potential is supplied isomitted.

In addition, a signal is supplied to each of the wiring 2603, the wiring2604, and the wiring 2605. Note that the signal supplied to each of thewiring 2603, the wiring 2604, and the wiring 2605 is a binary digitalsignal.

Note also that the power supply potential VDD, the power supplypotential VSS, or another power supply potential may be supplied to eachof the wiring 2603, the wiring 2604, and the wiring 2605. Alternatively,an analog signal may be supplied to each of the wiring 2603, the wiring2604, and the wiring 2605.

Next, operations of the basic circuit shown in FIG. 26A are describedwith reference to FIG. 26B. Note that FIG. 26B shows the case in whichthe basic circuits shown in FIGS. 16A, 17A, 20A, and 20B are used as thecircuit 2601 and the circuit 2602.

FIG. 26B is an example of a timing chart of the basic circuit shown inFIG. 26A. The timing chart in FIG. 26B shows a potential of the wiring2603, a potential of the wiring 2604, a potential of the wiring 2605,whether the output of the circuit 2601 is in a floating state (describedas OFF) or at the power supply potential VSS (described as ON), whetherthe output of the circuit 2602 is in a floating state (described as OFF)or at the power supply potential VSS (described as ON), and a potentialof the wiring 2606.

The liming chart in FIG. 26B is described by dividing the whole periodinto periods T1 to T8.

First, an operation in the period T1 is described. In the period T1, anH-level signal is supplied to the wiring 2605, an H-level signal issupplied to the wiring 2603, and an H-level signal is supplied to thewiring 2604. Each of the circuit 2601 and the circuit 2602 supplies thepower supply potential VDD to the wiring 2606. Therefore, the potentialof the wiring 2606 becomes equal to the power supply potential VDD.

Next, an operation in the period T2 is described. In the period T2, anH-level signal is supplied to the wiring 2605, an L-level signal issupplied to the wiring 2603, and an H-level signal is supplied to thewiring 2604. The circuit 2601 supplies no potential to the wiring 2606and the circuit 2602 supplies the power supply potential VDD to thewiring 2606. Therefore, the potential of the wiring 2606 becomes equalto the power supply potential VDD.

Next, an operation in the period T3 is described. In the period T3, anH-level signal is supplied to the wiring 2605, an H-level signal issupplied to the wiring 2603, and an L-level signal is supplied to thewiring 2604. The circuit 2601 supplies the power supply potential VDD tothe wiring 2606 and the circuit 2602 supplies no potential to the wiring2606. Therefore, the potential of the wiring 2606 becomes equal to thepower supply potential VDD.

Next, an operation in the period T4 is described. In the period T4, anH-level signal is supplied to the wiring 2605, an L-level signal issupplied to the wiring 2603, and an L-level signal is supplied to thewiring 2604. Each of the circuit 2601 and the circuit 2602 supplies nopotential to the wiring 2606. Therefore, the potential of the wiring2606 remains equal to the power supply potential VDD because the wiring2606 is kept at the potential in the period T3.

Next, an operation in the period T5 is described. In the period T5, anL-level signal is supplied to the wiring 2605, an H-level signal issupplied to the wiring 2603, and an H-level signal is supplied to thewiring 2604. Each of the circuit 2601 and the circuit 2602 supplies nopotential to the wiring 2606. Therefore, the potential of the wiring2606 remains equal to the power supply potential VDD because the wiring2606 is kept at the potential in the period T3.

Next, an operation in the period T6 is described. In the period T6, anL-level signal is supplied to the wiring 2605, an L-level signal issupplied to the wiring 2603, and an H-level signal is supplied to thewiring 2604. Each of the circuit 2601 and the circuit 2602 supplies nopotential to the wiring 2606. Therefore, the potential of the wiring2606 remains equal to the power supply potential VDD because the wiring2606 is kept at the potential in the period T3.

Next, an operation in the period T7 is described. In the period T7, anL-level signal is supplied to the wiring 2605, an H-level signal issupplied to the wiring 2603, and an L-level signal is supplied to thewiring 2604. Each of the circuit 2601 and the circuit 2602 supplies nopotential to the wiring 2606. Therefore, the potential of the wiring2606 remains equal to the power supply potential VDD because the wiring2606 is kept at the potential in the period T3.

Next, an operation in the period T8 is described. In the period T8, anL-level signal is supplied to the wiring 2605, an L-level signal issupplied to the wiring 2603, and an L-level signal is supplied to thewiring 2604. Each of the circuit 2601 and the circuit 2602 supplies nopotential to the wiring 2606. Therefore, the potential of the wiring2606 remains equal to the power supply potential VDD because the wiring2606 is kept at the potential in the period T3.

By the above-described operations, each of the circuit 2601 and thecircuit 2602 supplies the power supply potential VDD to the wiring 2606in the period T1, so that the potential of the wiring 2606 becomes equalto the power supply potential VDD. In the period T2, the circuit 2602supplies the power supply potential VDD to the wiring 2606, so that thepotential of the wiring 2606 becomes equal to the power supply potentialVDD. In the period T3, the circuit 2601 supplies the power supplypotential VDD to the wiring 2606, so that the potential of the wiring2606 becomes equal to the power supply potential VDD. In the periods T4to T8, the wiring 2606 is made into a floating state, so that thepotential of the wiring 2606 is kept equal to the power supply potentialVDD.

In addition, the basic circuit in FIG. 26A does not include a transistorwhich is on in all of the periods T1 to T8. That is, the basic circuitin FIG. 26A does not include a transistor which is always or almostalways on. Accordingly, the basic circuit in FIG. 26A can suppresscharacteristic deterioration of a transistor and a threshold voltageshift due to the characteristic deterioration.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 5

In this embodiment mode, the case is described in which the basiccircuit described in Embodiment Mode 1 is applied to a flip-flopcircuit, with reference to FIG. 27.

FIG. 27 is an example of a flip-flop circuit to which the basic circuitin FIG. 1A described in Embodiment Mode 1 is applied. The flip-flopcircuit in FIG. 27 includes a transistor 2701, a transistor 2702, atransistor 2703, a transistor 2704, a transistor 2705, a transistor2706, a transistor 2707, and a transistor 2708.

Note that the transistor 2705 corresponds to the transistor 101 in FIG.1A; the transistor 2707 corresponds to the transistor 103 in FIG. 1A,and the transistor 2706 corresponds to the transistor 102 in FIG. 1A. Inaddition, the transistor 2703 and the transistor 2704 correspond to thetransistor 104 in FIG. 1A.

Connection relations of the flip-flop circuit in FIG. 27 are described.Note that a node of a second terminal of the transistor 2701, a secondterminal of the transistor 2708, a gate of the transistor 2706, a secondterminal of the transistor 2704, and a gate of the transistor 2702 isdenoted by N271. In addition, a node of a second terminal of thetransistor 2705, a second terminal of the transistor 2706, a secondterminal of the transistor 2707, a gate of the transistor 2703, and agate of the transistor 2704 is denoted by N272.

A gate of the transistor 2701 is connected to a wiring 2712, a firstterminal of the transistor 2701 is connected to a wiring 2709, and thesecond terminal of the transistor 2701 is connected to the node N271. Agate of the transistor 2708 is connected to a wiring 2713, a firstterminal of the transistor 2708 is connected to a wiring 2710, and thesecond terminal of the transistor 2708 is connected to the node N271.Agate of the transistor 2705 is connected to the wiring 2709, a firstterminal of the transistor 2705 is connected to the wiring 2709, and thesecond terminal of the transistor 2705 is connected to the node N272. Agate of the transistor 2706 is connected to the node N271, a firstterminal of the transistor 2706 is connected to the wiring 2710, and thesecond terminal of the transistor 2706 is connected to the node N272. Agate of the transistor 2707 is connected to a wiring 2711, a firstterminal of the transistor 2707 is connected to the wiring 2710, and thesecond terminal of the transistor 2707 is connected to the node N272.The gate of the transistor 2704 is connected to the node N272, a firstterminal of the transistor 2704 is connected to the wiring 2710, and thesecond terminal of the transistor 2704 is connected to the node N271.The gate of the transistor 2703 is connected to the node N272, a firstterminal of the transistor 2703 is connected to the wiring 2710, and asecond terminal of the transistor 2703 is connected to a wiring 2714.The gate of the transistor 2702 is connected to the node N271, a firstterminal of the transistor 2702 is connected to the wiring 2711, and asecond terminal of the transistor 2702 is connected to the wiring 2714.

In addition, each of the transistors 2701 to 2708 is an N-channeltransistor.

Accordingly, since the flip-flop circuit in FIG. 27 can be formed byusing only N-channel transistors, amorphous silicon can be used for asemiconductor layer of the flip-flop circuit in FIG. 27. Thus, amanufacturing process can be simplified, so that manufacturing cost canbe reduced and a yield can be improved. In addition, a semiconductordevice such as a large display panel can also be formed. Further, whenpolysilicon or single crystalline silicon is used for the semiconductorlayer of the flip-flop circuit in FIG. 27, the manufacturing process canalso be simplified.

In addition, the power supply potential VDD is supplied to the wiring2709 and the power supply potential VSS is supplied to the wiring 2710.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 2709 and the wiring 2710, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 2711, the wiring2712, and the wiring 2713. Note that the signal supplied to each of thewiring 2711, the wiring 2712, and the wiring 2713 is a binary digitalsignal. Note also that the power supply potential VDD, the power supplypotential VSS, or another power supply potential may be supplied to eachof the wiring 2711, the wiring 2712, and the wiring 2713. Alternatively,an analog signal may be supplied to each of the wiring 2711, the wiring2712, and the wiring 2713.

Next, operations of the flip-flop circuit shown in FIG. 27 are describedwith reference to FIG. 28.

FIG. 28 is an example of a timing chart of the flip-flop circuit shownin FIG. 27. The timing chart in FIG. 28 shows a potential of the wiring2711, a potential of the wiring 2712, a potential of the node N271, apotential of the node N272, a potential of the wiring 2714, a relationof on/off of the transistor 2703 and the transistor 2704, and apotential of the wiring 2713.

The timing chart in FIG. 28 is described by dividing the whole periodinto periods T1 to T4. In addition, the period T3 is described bydividing the whole period into a period T3 a and a period T3 b. Further,FIGS. 29 to 33 show operations of the flip-flop circuit in FIG. 27 inthe periods T1, T2, T3 b, T4, and T3 a, respectively.

Note that the period T3 a and the period T4 are sequentially repeated inthe periods other than the periods T1, T2, and T3 b.

First, an operation in the period T1 is described with reference to FIG.29. In the period T1, an L-level signal is supplied to the wiring 2711,an H-level signal is supplied to the wiring 2712, and an L-level signalis supplied to the wiring 2713.

Accordingly, the transistor 2701 is turned on and the transistor 2708and the transistor 2707 are turned off. At this time, the power supplypotential VDD is supplied to the node N271 through the transistor 2701,so that the potential of the node N271 rises. In addition, thetransistor 2706 is turned on by the rise in the potential of the nodeN271, so that the potential of the node N272 decreases. Further, thetransistor 2703 and the transistor 2704 are turned off by the decreasein the potential of the node N272.

Here, the rise in the potential of the node N271 continues until thetransistor 2701 is turned off. The transistor 2701 is turned off whenthe potential of the node N271 becomes a value obtained by subtracting athreshold voltage Vth2701 of the transistor 2701 from the power supplypotential VDD (VDD−Vth2701). Therefore, the potential of the node N271becomes VDD−Vth2701. In addition, the node N271 becomes a floatingstate.

Therefore, the transistor 2702 is turned on. In addition, since theL-level signal of the wiring 2711 is supplied to the wiring 2714, thepotential of the wiring 2714 becomes equal to the power supply potentialVSS.

Next, an operation in the period T2 is described with reference to FIG.30. In the period T2, an H-level signal is supplied to the wiring 2711,an L level signal is supplied to the wiring 2712, and an L-level signalis supplied to the wiring 2713.

Accordingly, the transistor 2701 is turned off, the transistor 2708 iskept off, and the transistor 2707 is turned on. At this time, the nodeN271 is in a floating state, and the potential of the node N271 is keptat VDD-Vth2701. In addition, the potential of the node N272 remains atan L level because the transistor 2706 and the transistor 2707 are on.Thus, since the node N272 is at the L level, the transistor 2703 and thetransistor 2704 are kept off.

Here, the node N271 is in a floating state and kept at an H level. Inaddition, since the node N271 is kept at the H level, the transistor2702 is kept on. Further, since the H-level signal of the wiring 2711 issupplied to the wiring 2714, the potential of the wiring 2714 rises.Therefore, the potential of the node N271 becomes equal to or higherthan the sum of the power supply potential VDD and a threshold voltageVth2702 of the transistor 2702 (VDD+Vth2702) by a bootstrap operation,so that the potential of the wiring 2714 becomes equal to the powersupply potential VDD.

Next, an operation in the period T3 b is described with reference toFIG. 31. In the period T3 b, an L-level signal is supplied to the wiring2711, an L-level signal is supplied to the wiring 2712, and an H-levelsignal is supplied to the wiring 2713.

Accordingly, the transistor 2701 is kept off, the transistor 2708 isturned on, and the transistor 2707 is turned off. At this time, thepower supply potential VSS is supplied to the node N271 through thetransistor 2708, so that the potential of the node N271 decreases. Inaddition, the transistor 2706 is turned off by the decrease in thepotential of the node N271, so that the potential of the node N272rises. Further, the transistor 2703 and the transistor 2704 are turnedon by the rise in the potential of the node N272.

In addition, the transistor 2702 is turned off by the decrease in thepotential of the node N271. Therefore, since the power supply potentialVSS is supplied to the wiring 2714 through the transistor 2703, thepotential of the wiring 2714 becomes equal to the power supply potentialVSS.

Next, an operation in the period T4 is described with reference to FIG.32. In the period T4, an H-level signal is supplied to the wiring 2711,an L-level signal is supplied to the wiring 2712, and an L-level signalis supplied to the wiring 2713.

Accordingly, the transistor 2701 is kept off, the transistor 2708 isturned off, and the transistor 2707 is turned on. At this time, the nodeN271 becomes a floating state, and the potential of the node N271 iskept at the power supply potential VSS. Thus, the transistor 2706 andthe transistor 2702 are turned off. In addition, the potential of thenode N272 becomes an L level because the power supply potential VSS issupplied thereto through the transistor 2707. Therefore, the transistor2703 and the transistor 2704 are turned off.

Therefore, the wiring 2714 becomes a floating state, and the potentialof the wiring 2714 is kept equal to the power supply potential VSS.

Next, an operation in the period T3 a is described with reference toFIG. 33. In the period T3 a, an L-level signal is supplied to the wiring2711, an L-level signal is supplied to the wiring 2712, and an L-levelsignal is supplied to the wiring 2713.

Accordingly, the transistor 2701 and the transistor 2708 are kept off,and the transistor 2707 is turned off. At this time, since thetransistor 2707 is turned off, the potential of the node N272 rises.Thus, the transistor 2703 and the transistor 2704 are turned on. Inaddition, the power supply potential VSS is supplied to the node N271through the transistor 2704, so that the potential of the node N271becomes equal to the power supply potential VSS. Therefore, thetransistor 2702 and the transistor 2706 are kept off.

Further, the power supply potential VSS is supplied to the wiring 2714through the transistor 2703, and the potential of the wiring 2714 iskept equal to the power supply potential VSS.

By the above-described operations, the flip-flop circuit in FIG. 27keeps the node N271 at an H level to be in a floating state in theperiod T1. In the period T2, the flip-flop circuit in FIG. 27 sets thepotential of the node N271 to be equal to or higher than VDD+Vth2702 bythe bootstrap operation, so that the potential of the wiring 2714 can beset equal to the power supply potential VDD.

Further, in the period T3 a, the flip-flop circuit in FIG. 27 turns onthe transistor 2703 and the transistor 2704, and supplies the powersupply potential VSS to the wiring 2714 and the node N271. In the periodT4, the flip-flop circuit in FIG. 27 turns off the transistor 2703 andthe transistor 2704. Therefore, since the flip-flop circuit in FIG. 27sequentially turns on the transistor 2703 and the transistor 2704, itcan suppress characteristic deterioration of the transistor 2703 and thetransistor 2704, so that the potential of each of the node N271 and thewiring 2714 can be stably kept equal to the power supply potential VSS.

In addition, the flip-flop circuit in FIG. 27 does not include atransistor which is on in all of the periods T1 to T4. That is, theflip-flop circuit in FIG. 27 does not include a transistor which isalways or almost always on. Accordingly, the flip-flop circuit in FIG.27 can suppress characteristic deterioration of a transistor and athreshold voltage shift due to the characteristic deterioration.

Further, the characteristics of a transistor which is formed ofamorphous silicon easily deteriorate. Therefore, when the transistorincluded in the flip-flop circuit in FIG. 27 is formed using amorphoussilicon, not only can the advantages such as a reduction inmanufacturing cost and improvement in a yield be obtained, but also theproblem of the characteristic deterioration of the transistor can besolved.

Here, the functions of the transistors 2701 to 2708 are described. Thetransistor 2701 has a function of a switch which selects whether toconnect the wiring 2709 and the node N271 in accordance with thepotential of the wiring 2712. The transistor 2702 has a function of aswitch which selects whether to connect the wiring 2711 and the wiring2714 in accordance with the potential of the node N271. The transistor2703 has a function of a switch which selects whether to connect thewiring 2710 and the wiring 2714 in accordance with the potential of thenode N272. The transistor 2704 has a function of a switch which selectswhether to connect the wiring 2710 and the node N271 in accordance withthe potential of the node N272. The transistor 2705 has a function of adiode in which the first terminal and the gate correspond to an inputterminal and the second terminal corresponds to an output terminal. Thetransistor 2706 has a function of a switch which selects whether toconnect the wiring 2710 and the node N272 in accordance with thepotential of the node N271. The transistor 2707 has a function of aswitch which selects whether to connect the wiring 2710 and the nodeN272 in accordance with the potential of the wiring 2711. The transistor2708 has a function of a switch which selects whether to connect thewiring 2710 and the node N271 in accordance with the potential of thewiring 2713.

Note that a two-input NOR circuit in which the node N271 and the wiring2711 correspond to an input terminal and the node N272 corresponds to anoutput terminal is constructed from the transistor 2705, the transistor2706, and the transistor 2707.

Note that the transistor 2705 may be any element as long as it has aresistance component. For example, as shown in FIG. 34, a resistor 3401can be used instead of the transistor 2705. By using the resistor 3401,the potential of the node N272 can be set equal to the power supplypotential VDD.

Note that as shown in FIG. 35, a capacitor 3501 may be provided betweenthe gate (the node N271) and the second terminal (the wiring 2714) ofthe transistor 2702. This is because the potential of the node N271 andthe potential of the wiring 2714 are raised by the bootstrap operationin the period T2, so that the flip-flop circuit can easily perform thebootstrap operation by proving the capacitor 3501.

Note that it is only necessary that the transistor 2701 make the nodeN271 into a floating state in the period T1 so that the potential of thenode N271 becomes an H level. Therefore, even when the first terminal ofthe transistor 2701 is connected to the wiring 2712, the transistor 2701can make the node N271 into a floating state so that the potential ofthe node N271 becomes an H level.

Next, the case is described in which the flip-flop circuit shown in FIG.27 is constructed from P-channel transistors, with reference to FIG. 44.

FIG. 44 is an example of a flip-flop circuit to which the basic circuitin FIG. 13A described in Embodiment Mode 1 is applied. The flip-flopcircuit in FIG. 44 includes a transistor 4401, a transistor 4402, atransistor 4403, a transistor 4404, a transistor 4405, a transistor4406, a transistor 4407, and a transistor 4408.

Note that the transistor 4405 corresponds to the transistor 1301 in FIG.13A, the transistor 4407 corresponds to the transistor 1302 in FIG. 13A,and the transistor 4406 corresponds to the transistor 1303 in FIG. 13A.In addition, the transistor 4403 and the transistor 4404 correspond tothe transistor 1304 in FIG. 13A.

Connection relations of the flip-flop circuit in FIG. 44 are described.Note that a node of a second terminal of the transistor 4401, a secondterminal of the transistor 4408, a gate of the transistor 4406, a secondterminal of the transistor 4404, and a gate of the transistor 4402 isdenoted by N441. In addition, a node of a second terminal of thetransistor 4405, a second terminal of the transistor 4406, a secondterminal of the transistor 4407, a gate of the transistor 4403, and agate of the transistor 4404 is denoted by N442.

A gate of the transistor 4401 is connected to a wiring 4412, a firstterminal of the transistor 4401 is connected to a wiring 4409, and thesecond terminal of the transistor 4401 is connected to the node N441. Agate of the transistor 4408 is connected to a wiring 4413, a firstterminal of the transistor 4408 is connected to a wiring 4410, and thesecond terminal of the transistor 4408 is connected to the node N441. Agate of the transistor 4405 is connected to the wiring 4409, a firstterminal of the transistor 4405 is connected to the wiring 4409, and thesecond terminal of the transistor 4405 is connected to the node N442. Agate of the transistor 4406 is connected to the node N441, a firstterminal of the transistor 4406 is connected to the wiring 4410, and thesecond terminal of the transistor 4406 is connected to the node N442. Agate of the transistor 4407 is connected to a wiring 4411, a firstterminal of the transistor 4407 is connected to the wiring 4410, and thesecond terminal of the transistor 4407 is connected to the node 442. Thegate of the transistor 4404 is connected to the node N442, a firstterminal of the transistor 4404 is connected to the wiring 4410, and thesecond terminal of the transistor 4404 is connected to the node N441.The gate of the transistor 4403 is connected to the node N442, a firstterminal of the transistor 4403 is connected to the wiring 4410, and asecond terminal of the transistor 4403 is connected to a wiring 4414.The gate of the transistor 4402 is connected to the node N441, a firstterminal of the transistor 4402 is connected to the wiring 4411, and asecond terminal of the transistor 4402 is connected to the wiring 4414.

In addition, each of the transistors 4401 to 4408 is a P-channeltransistor.

Accordingly, since the flip-flop circuit in FIG. 44 can be formed byusing only P-channel transistors, a step of forming N-channeltransistors is not necessary. Thus, in the flip-flop circuit in FIG. 44,a manufacturing process can be simplified, so that manufacturing costcan be reduced and a yield can be improved.

In addition, the power supply potential VDD is supplied to the wiring4410 and the power supply potential VSS is supplied to the wiring 4409.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 4409 and the wiring 4410, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 4411, the wiring4412, and the wiring 4413. Note that the signal supplied to each of thewiring 4411, the wiring 4412, and the wiring 4413 is a binary digitalsignal. Note also that the power supply potential VDD, the power supplypotential VSS, or another power supply potential may be supplied to eachof the wiring 4411, the wiring 4412, and the wiring 4413. Alternatively,an analog signal may be supplied to each of the wiring 4411, the wiring4412, and the wiring 4413.

Next, operations of the flip-flop circuit shown in FIG. 44 are describedwith reference to FIG. 45.

FIG. 45 is an example of a timing chart of the flip-flop circuit shownin FIG. 44. The timing chart in FIG. 45 shows a potential of the wiring4411, a potential of the wiring 4412, a potential of the node N441, apotential of the node N442, a potential of the wiring 4414, a relationof on/off of the transistor 4403 and the transistor 4404, and apotential of the wiring 4413.

The timing chart in FIG. 44 is described by dividing the whole periodinto periods T1 to T4. In addition, the period T3 is described bydividing the whole period into a period T3 a and a period T3 b.

Note that the period T3 a and the period T4 are sequentially repeated inthe periods other than the periods T1, T2, and T3 b.

First, an operation in the period T1 is described. In the period T1, anH-level signal is supplied to the wiring 4411, an L-level signal issupplied to the wiring 4412, and an H-level signal is supplied to thewiring 4413.

Accordingly, the transistor 4401 is turned on and the transistor 4408and the transistor 4407 are turned off. At this time, the power supplypotential VSS is supplied to the node N441 through the transistor 4401,so that the potential of the node N441 decreases. In addition, thetransistor 4406 is turned on by the decrease in the potential of thenode N441, so that the potential of the node N442 rises. Further, thetransistor 4403 and the transistor 4404 are turned off by the rise inthe potential of the node N442.

Here, the decrease in the potential of the node N441 continues until thetransistor 4401 is turned off. The transistor 4401 is turned off whenthe potential of the node N441 becomes the sum of the power supplypotential VSS and the absolute value of a threshold voltage Vth4401 ofthe transistor 4401 (VSS+|Vth4401|). Therefore, the potential of thenode N441 becomes VSS+|Vth4401|. In addition, the node N441 becomes afloating state.

Therefore, the transistor 4402 is turned on. In addition, since theH-level signal of the wiring 4411 is supplied to the wiring 4414, thepotential of the wiring 4414 becomes equal to the power supply potentialVDD.

Next, an operation in the period T2 is described. In the period T2, anb-level signal is supplied to the wiring 4411, an H-level signal issupplied to the wiring 4412, and an H-level signal is supplied to thewiring 4413.

Accordingly, the transistor 4401 is turned off, the transistor 4408 iskept off, and the transistor 4407 is turned on. At this time, the nodeN441 is in a floating state, and the potential of the node N441 is keptat VSS+|Vth4401|. In addition, the potential of the node N442 remains atan H level because the transistor 4406 and the transistor 4407 are on.Thus, since the node N442 is at the H level, the transistor 4403 and thetransistor 4404 are kept off.

Here, the node N441 is in a floating state and kept at an L level. Inaddition, since the node N441 is kept at the L level, the transistor4402 is kept on. Further, since the L-level signal of the wiring 4411 issupplied to the wiring 4414, the potential of the wiring 4414 decreases.Therefore, the potential of the node N441 becomes equal to or lower thana value obtained by subtracting the absolute value of a thresholdvoltage Vth4402 of the transistor 4402 from the power supply potentialVSS (VSS-|Vth4402|) by a bootstrap operation, so that the potential ofthe wiring 4414 becomes equal to the power supply potential VSS.

Next, an operation in the period T3 b is described. In the period T3 b,an H-level signal is supplied to the wiring 4411, an H-level signal issupplied to the wiring 4412, and an L-level signal is supplied to thewiring 4413.

Accordingly, the transistor 4401 is kept off, the transistor 4408 isturned on, and the transistor 4407 is turned off. At this time, thepower supply potential VDD is supplied to the node N441 through thetransistor 4408, so that the potential of the node N441 rises. Inaddition, the transistor 4406 is turned off by the rise in the potentialof the node N441, so that the potential of the node N442 decreases.Further, the transistor 4403 and the transistor 4404 are turned on bythe decrease in the potential of the node N442.

In addition, the transistor 4402 is turned off by the rise in thepotential of the node N441. Therefore, since the power supply potentialVDD is supplied to the wiring 4414 through the transistor 4403, thepotential of the wiring 4414 becomes equal to the power supply potentialVDD.

Next, an operation in the period T4 is described. In the period T4, anL-level signal is supplied to the wiring 4411, an H-level signal issupplied to the wiring 4412, and an H-level signal is supplied to thewiring 4413.

Accordingly, the transistor 4401 is kept off, the transistor 4408 isturned off, and the transistor 4407 is turned on. At this time, the nodeN441 becomes a floating state, and the potential of the node N441 iskept at the power supply potential VDD. Thus, the transistor 4406 andthe transistor 4402 are turned off. In addition, the potential of thenode N442 becomes an H level because the power supply potential VDD issupplied thereto through the transistor 4407. Therefore, the transistor4403 and the transistor 4404 are turned off.

Therefore, the wiring 4414 becomes a floating state, and the potentialof the wiring 4414 is kept equal to the power supply potential VDD.

Next, an operation in the period T3 a is described. In the period T3 a,an H-level signal is supplied to the wiring 4411, an H-level signal issupplied to the wiring 4412, and an H-level signal is supplied to thewiring 4413.

Accordingly, the transistor 4401 and the transistor 4408 are kept off,and the transistor 4407 is turned off. At this time, since thetransistor 4407 is turned off, the potential of the node N442 decreases.Thus, the transistor 4403 and the transistor 4404 are turned on. Inaddition, the power supply potential VDD is supplied to the node N441through the transistor 4404, so that the potential of the node N441becomes equal to the power supply potential VDD. Therefore, thetransistor 4402 and the transistor 4406 are kept off.

Further, the power supply potential VDD is supplied to the wiring 4414through the transistor 4403, and the potential of the wiring 4414 iskept equal to the power supply potential VDD.

By the above-described operations, the flip-flop circuit in FIG. 44keeps the node N441 at an H level to be in a floating state in theperiod T1. In the period T2, the flip-flop circuit in FIG. 44 sets thepotential of the node N441 equal to or lower than VSS−|Vth4402| by thebootstrap operation, so that the potential of the wiring 4414 can be setequal to the power supply potential VSS.

Further, in the period T3 a, the flip-flop circuit in FIG. 44 turns onthe transistor 4403 and the transistor 4404, and supplies the powersupply potential VDD to the wiring 4414 and the node N441. In the periodT4, the flip-flop circuit in FIG. 44 turns off the transistor 4403 andthe transistor 4404. Therefore, since the flip-flop circuit in FIG. 44sequentially turns on the transistor 4403 and the transistor 4404, itcan suppress characteristic deterioration of the transistor 4403 and thetransistor 4404, so that the potential of each of the node N441 and thewiring 4414 can be stably kept equal to the power supply potential VDD.

In addition, the flip-flop circuit in FIG. 44 does not include atransistor which is on in all of the periods T1 to T4. That is, theflip-flop circuit in FIG. 44 does not include a transistor which isalways or almost always on. Accordingly, the flip-flop circuit in FIG.44 can suppress characteristic deterioration of a transistor and athreshold voltage shift due to the characteristic deterioration.

Note that the transistors 4401 to 4408 have functions which are similarto those of the transistors 2701 to 2708.

Note that a two-input NAND circuit in which the node N441 and thewirings 4411 correspond to an input terminal and the node N442corresponds to an output terminal is constructed from the transistors4405 to 4407.

Note that the transistor 4405 may be any element as long as it has aresistance component. For example, as shown in FIG. 46, a resistor 4601can be used instead of the transistor 4405. By using the resistor 4601,the potential of the node N442 can be set equal to the power supplypotential VSS.

Note that as shown in FIG. 47, a capacitor 4701 may be provided betweenthe gate (the node N441) and the second terminal (the wiring 4414) ofthe transistor 4402. This is because the potential of the node N441 andthe potential of the wiring 4414 are raised by the bootstrap operationin the period 12, so that the flip-flop circuit can easily perform thebootstrap operation by proving the capacitor 4701.

Note that it is only necessary that the transistor 4401 make the nodeN441 into a floating state in the period T1 so that the potential of thenode N441 becomes an L level. Therefore, even when the first terminal ofthe transistor 4401 is connected to the wiring 4412, the transistor 4401can make the node N441 into a floating state so that the potential ofthe node N441 becomes an L level.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 6

In this embodiment mode, the case is described in which the basiccircuit described in Embodiment Mode 2 is applied to a flip-flopcircuit, with reference to FIG. 36.

FIG. 36 is an example of a flip-flop circuit to which the basic circuitin FIG. 5A described in Embodiment Mode 2 is applied. The flip-flopcircuit in FIG. 36 includes a transistor 3600, a transistor 3601, atransistor 3602, a transistor 3603, a transistor 3604, a transistor3605, a transistor 3606, a transistor 3607, and a transistor 3608, atransistor 3609, and a transistor 3610.

Note that the transistor 3605 corresponds to the transistor 501 in FIG.5A, the transistor 3607 corresponds to the transistor 502 in FIG. 5A,the transistor 3606 corresponds to the transistor 503 in FIG. 5A, thetransistor 3608 corresponds to the transistor 504 in FIG. 5A, and thetransistor 3610 corresponds to the transistor 505 in FIG. 5A, and thetransistor 3609 corresponds to the transistor 506 in FIG. 5A. Inaddition, the transistor 3603 and the transistor 3604 correspond to thetransistor 507 in FIG. 5A.

Connection relations of the flip-flop circuit in FIG. 36 are described.Note that a node of a second terminal of the transistor 3601, a secondterminal of the transistor 3600, a gate of the transistor 3606, a secondterminal of the transistor 3604, and a gate of the transistor 3602 isdenoted by N361. In addition, a node of a second terminal of thetransistor 3605, a second terminal of the transistor 3606, a secondterminal of the transistor 3607, and a gate of the transistor 3608 isdenoted by N362. Further, a node of a second terminal of the transistor3609, a second terminal of the transistor 3608, a second terminal of thetransistor 3610, a gate of the transistor 3603, and a gate of thetransistor 3604 is denoted by N363.

A gate of the transistor 3601 is connected to a wiring 3614, a firstterminal of the transistor 3601 is connected to a wiring 3611, and thesecond terminal of the transistor 3601 is connected to the node N361. Agate of the transistor 3600 is connected to a wiring 3615, a firstterminal of the transistor 3600 is connected to a wiring 3612, and thesecond terminal of the transistor 3600 is connected to the node N361.The gate of the transistor 3606 is connected to the node N361, a firstterminal of the transistor 3606 is connected to the wiring 3612, and thesecond terminal of the transistor 3606 is connected to the node N362. Agate of the transistor 3605 is connected to the wiring 3611, a firstterminal of the transistor 3605 is connected to the wiring 3611, and thesecond terminal of the transistor 3605 is connected to the node N362. Agate of the transistor 3607 is connected to a wiring 3613, a firstterminal of the transistor 3607 is connected to the wiring 3612, and thesecond terminal of the transistor 3607 is connected to the node N362.The gate of the transistor 3608 is connected to the node N362, a firstterminal of the transistor 3608 is connected to the wiring 3611, and thesecond terminal of the transistor 3608 is connected to the node N363. Agate of the transistor 3609 is connected to the node N361, a firstterminal of the transistor 3609 is connected to the wiring 3612, and thesecond terminal of the transistor 3609 is connected to the node N363. Agate of the transistor 3610 is connected to the wiring 3613, a fastterminal of the transistor 3610 is connected to the wiring 3612, and thesecond terminal of the transistor 3610 is connected to the node N363.The gate of the transistor 3604 is connected to the node N363, a firstterminal of the transistor 3604 is connected to the wiring 3612, and thesecond terminal of the transistor 3604 is connected to the node N361.The gate of the transistor 3603 is connected to the node N363, a firstterminal of the transistor 3603 is connected to the wiring 3612, and asecond terminal of the transistor 3603 is connected to a wiring 3616.The gate of the transistor 3602 is connected to the node N361, a firstterminal of the transistor 3602 is connected to the wiring 3613, and asecond terminal of the transistor 3602 is connected to the wiring 3616.

In addition, each of the transistors 3600 to 3610 is an N-channeltransistor.

Accordingly, since the flip-flop circuit in FIG. 36 can be formed byusing only N-channel transistors, amorphous silicon can be used for asemiconductor layer of the flip-flop circuit in FIG. 36. Thus, amanufacturing process can be simplified, so that manufacturing cost canbe reduced and a yield can be improved. In addition, a semiconductordevice such as a large display panel can also be formed. Further, whenpolysilicon or single crystalline silicon is used for the semiconductorlayer of the flip-flop circuit in FIG. 36, the manufacturing process canalso be simplified.

In addition, the power supply potential VDD is supplied to the wiring3611 and the power supply potential VSS is supplied to the wiring 3612.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 3611 and the wiring 3612, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 3613, the wiring3614, and the wiring 3615. Note that the signal supplied to each of thewiring 3613, the wiring 3614, and the wiring 3615 is a binary digitalsignal. Note also that the power supply potential VDD, the power supplypotential VSS, or another power supply potential may be supplied to eachof the wiring 3613, the wiring 3614, and the wiring 3615. Alternatively,an analog signal may be supplied to each of the wiring 3613, the wiring3614, and the wiring 3615.

Next, operations of the flip-flop circuit shown in FIG. 36 are describedwith reference to FIG. 37.

FIG. 37 is an example of a timing chart of the flip-flop circuit shownin FIG. 36. The timing chart in FIG. 37 shows a potential of the wiring3613, a potential of the wiring 3614, a potential of the node N361, apotential of the node N362, a potential of the node N363, a potential ofthe wiring 3616, a relation of on/off of the transistor 3603 and thetransistor 3604, a potential of the wiring 3615.

The timing chart in FIG. 37 is described by dividing the whole periodinto periods T1 to T4. In addition, the period T3 is described bydividing the whole period into a period T3 a and a period T3 b.

Note that the period T3 a and the period T4 are sequentially repeated inthe periods other than the periods T1, T2, and T3 b.

First, an operation in the period T1 is described. In the period T1, anL-level signal is supplied to the wiring 3613, an H-level signal issupplied to the wiring 3614, and an L-level signal is supplied to thewiring 3615.

Accordingly, the transistor 3601 is turned on, and the transistor 3600,the transistor 3607, and the transistor 3610 are turned off. At thistime, the power supply potential VDD is supplied to the node N361through the transistor 3601, so that the potential of the node N361rises. In addition, the transistor 3606 and the transistor 3609 areturned on by the rise in the potential of the node N361, so that thepotentials of the node N362 and the node N363 decrease. Further, thetransistor 3608 is turned off by the decrease in the potential of thenode N362. Moreover, the transistor 3603 and the transistor 3604 areturned off by the decrease in the potential of the node N363.

Here, the rise in the potential of the node N361 continues until thetransistor 3601 is turned off. The transistor 3601 is turned off whenthe potential of the node N361 becomes a value obtained by subtracting athreshold voltage Vth3601 of the transistor 3601 from the power supplypotential VDD (VDD-Vth3601). Therefore, the potential of the node N361becomes VDD−Vth3601. In addition, the node N361 becomes a floatingstate.

Therefore, the transistor 3602 is turned on. In addition, since theL-level signal of the wiring 3613 is supplied to the wiring 3616, thepotential of the wiring 3616 becomes equal to the power supply potentialVSS.

Next, an operation in the period T2 is described. In the period 12, anH-level signal is supplied to the wiring 3613, an L-level signal issupplied to the wiring 3614, and an L-level signal is supplied to thewiring 3615.

Accordingly, the transistor 3601 is turned off, the transistor 3600 iskept off, and the transistor 3607 and the transistor 3610 are turned on.At this time, the node N361 is in a floating state, and the potential ofthe node N361 is kept at VDD-Vth3601. In addition, the potential of thenode N362 remains at an L level because the transistor 3606 and thetransistor 3607 are on. Further, the potential of the node N363 remainsat an L level because the transistor 3609 and the transistor 3610 arton. Thus, since the node N363 is at the L level, the transistor 3603 andthe transistor 3604 are kept off.

Here, the node N361 is in a floating state and kept at an H level. Inaddition, since the node N361 is kept at the H level, the transistor3602 is kept on. Further, since the H-level signal of the wiring 3613 issupplied to the wiring 3616, the potential of the wiring 3616 rises.Therefore, the potential of the node N361 becomes equal to or higherthan the sum of the power supply potential VDD and a threshold voltageVth3602 of the transistor 3602 (VDD+Vth3602) by a bootstrap operation,so that the potential of the wiring 3616 becomes equal to the powersupply potential VDD.

Next, an operation in the period T3 b is described. In the period T3 b,an L-level signal is supplied to the wiring 3613, an L-level signal issupplied to the wiring 3614, and an H-level signal is supplied to thewiring 3615.

Accordingly, the transistor 3601 is kept off, the transistor 3600 isturned on, and the transistor 3607 and the transistor 3610 are turnedoff. At this time, the power supply potential VSS is supplied to thenode N361 through the transistor 3600, so that the potential of the nodeN361 decreases. In addition, the transistor 3606 and the transistor 3607are turned off by the decrease in the potential of the node N361.Therefore, the potentials of the node N362 and the node N363 are raisedby a bootstrap operation. The potential of the node N362 rises to beequal to or higher than the sum of the power supply potential VDD and athreshold voltage Vth3608 of the transistor 3608 (VDD+Vth3608). Thepotential of the node N363 rises to the power supply potential VDD.Therefore, the transistor 3603 and the transistor 3604 are turned on bythe rise in the potential of the node N363.

In addition, the transistor 3602 is turned off by the decrease in thepotential of the node N361. Therefore, since the power supply potentialVSS is supplied to the wiring 3616 through the transistor 3603, thepotential of the wiring 3616 becomes equal to the power supply potentialVSS.

Next, an operation in the period T4 is described. In the period T4, anH-level signal is supplied to the wiring 3613, an L-level signal issupplied to the wiring 3614, and an L-level signal is supplied to thewiring 3615.

Accordingly, the transistor 3601 is kept off, the transistor 3600 isturned off, and the transistor 3607 and the transistor 3610 are turnedon. At this time, the node N361 is in a floating state, and thepotential of the node N361 is kept at the power supply potential VSS.Thus, the transistors 3602, 3606, and 3609 are kept off. In addition,the potential of the node N362 becomes an L level because the powersupply potential VSS is supplied thereto through the transistor 3607.Further, the potential of the node N363 becomes an L level because thepower supply potential VSS is supplied thereto through the transistor3610. Therefore, the transistor 3603 and the transistor 3604 are turnedoff.

Therefore, the wiring 3616 becomes a floating state, and the potentialof the wiring 3616 is kept equal to the power supply potential VSS.

Next, an operation in the period T3 a is described. In the period T3 a,an L-level signal is supplied to the wiring 3613, an L-level signal issupplied to the wiring 3614, and an L-level signal is supplied to thewiring 3615.

Accordingly, the transistor 3601 and the transistor 3600 are kept off,and the transistor 3607 and the transistor 3610 are turned off. At thistime, the node N361 is in a floating state, and the potential of thenode N361 remains at an L level. Thus, the transistors 3602, 3606, and3609 are kept off. In addition, the potentials of the node N362 and thenode N363 are raised by a bootstrap operation. The potential of the nodeN362 rises to be equal to or higher than the sum of the power supplypotential VDD and the threshold voltage Vth3608 of the transistor 3608(VDD+Vth3608). The potential of the node N363 rises to the power supplypotential VDD. Therefore, the transistor 3603 and the transistor 3604are turned on by the rise in the potential of the node N363.

Therefore, since the power supply potential VSS is supplied to thewiring 3616 through the transistor 3603, the potential of the wiring3616 is kept equal to the power supply potential VSS.

By the above-described operations, the flip-flop circuit in FIG. 36keeps the node N361 at an H level to be in a floating state in theperiod T1. In the period T2, the flip-flop circuit in FIG. 36 sets thepotential of the node N361 equal to or higher than VDD+Vth3602 by thebootstrap operation, so that the potential of the wiring 3616 is madeequal to the power supply potential VDD.

Further, in the period T3 a, the flip-flop circuit in FIG. 36 turns onthe transistor 3603 and the transistor 3604, and supplies the powersupply potential VSS to the wiring 3616 and the node N361. In the periodT4, the flip-flop circuit in FIG. 36 turns off the transistor 3603 andthe transistor 3604. Therefore, since the flip-flop circuit in FIG. 36sequentially turns on the transistor 3603 and the transistor 3604, itcan suppress characteristic deterioration of the transistor 3603 and thetransistor 3604, so that the potential of each of the node N361 and thewiring 3616 can be stably kept equal to the power supply potential VSS.

In addition, the flip-flop circuit in FIG. 36 can set the potential ofthe node N363 to be equal to the power supply potential VDD in theperiods T3 a and T3 b. Therefore, even when the characteristics of thetransistor 3603 and the transistor 3604 deteriorate, the flip-flopcircuit in FIG. 36 can be operated under a wide range of operatingconditions.

In addition, the flip-flop circuit in FIG. 36 does not include atransistor which is on in all of the periods T1 to T4. That is, theflip-flop circuit in FIG. 36 does not include a transistor which isalways or almost always on. Accordingly, the flip-flop circuit in FIG.36 can suppress characteristic deterioration of a transistor and athreshold voltage shift due to the characteristic deterioration.

Further, the characteristics of a transistor which is formed ofamorphous silicon easily deteriorate. Therefore, when the transistorincluded in the flip-flop circuit in FIG. 36 is formed using amorphoussilicon, not only can the advantages such as a reduction inmanufacturing cost and improvement in a yield be obtained, but also theproblem of the characteristic deterioration of the transistor can besolved.

Here, the functions of the transistors 3600 to 3610 are described. Thetransistor 3600 has a function of a switch which selects whether toconnect the wiring 3612 and the node N361 in accordance with thepotential of the wiring 3615. The transistor 3601 has a function of aswitch which selects whether to connect the wiring 3611 and the nodeN361 in accordance with the potential of the wiring 3614. The transistor3602 has a function of a switch which selects whether to connect thewiring 3613 and the wiring 3616 in accordance with the potential of thenode N361. The transistor 3603 has a function of a switch which selectswhether to connect the wiring 3612 and the wiring 3616 in accordancewith the potential of the node N363. The transistor 3604 has a functionof a switch which selects whether to connect the wiring 3612 and thenode N361 in accordance with the potential of the node N363. Thetransistor 3605 has a function of a diode in which the first terminaland the gate correspond to an input terminal and the second terminalcorresponds to an output terminal. The transistor 3606 has a function ofa switch which selects whether to connect the wiring 3612 and the nodeN362 in accordancc with the potential of the node N361. The transistor3607 has a function of a switch which selects whether to connect thewiring 3612 and the node N362 in accordance with the potential of thewiring 3613. The transistor 3608 has a function of a switch whichselects whether to connect the wiring 3611 and the node N363 inaccordance with the potential of the node N362. The transistor 3609 hasa function of a switch which selects whether to connect the wiring 3612and the node N363 in accordance with the potential of the node N361. Thetransistor 3610 has a function of a switch which selects whether toconnect the wiring 3612 and the node N363 in accordance with thepotential of the wiring 3613.

Note that a two-input NOR circuit in which the node N361 and the wiring3613 correspond to an input terminal and the node N363 corresponds to anoutput terminal is constructed from the transistors 3605 to 3610.

Note that as shown in FIG. 38, a capacitor 3801 may be provided betweenthe gate (the node N362) and the second terminal (the node N363) of thetransistor 3608. This is because the potential of the node N362 and thepotential of the node N363 are raised by the bootstrap operation in theperiods T3 a and T3 b, so that the flip-flop circuit can easily performthe bootstrap operation by proving the capacitor 3801.

Note that as shown in FIG. 39, the transistor 3607 is not necessarilyprovided.

Note that as shown in FIG. 40, a capacitor 4111 may be provided betweenthe gate (the node N361) and the second terminal (the wiring 3616) ofthe transistor 3602. This is because the potential of the node N361 andthe potential of the wiring 3616 are raised by the bootstrap operationin the period T2, so that the flip-flop circuit can easily perform thebootstrap operation by proving the capacitor 4111.

Note that it is only necessary that the transistor 3601 make the nodeN361 into a floating state in the period T1 so that the potential of thenode N361 becomes an H level. Therefore, even when the first terminal ofthe transistor 3601 is connected to the wiring 3614, the transistor 3601can make the node N361 into a floating state so that the potential ofthe node N361 becomes an H level.

Next, the case is described in which the flip-flop circuit shown in FIG.36 is constructed from P-channel transistors, with reference to FIG. 48.

FIG. 48 is an example of a flip-flop circuit to which the basic circuitin FIG. 17A described in Embodiment Mode 2 is applied. The flip-flopcircuit in FIG. 48 includes a transistor 4800, transistor 4801, atransistor 4802, a transistor 4803, a transistor 4804, a transistor4805, a transistor 4806, a transistor 4807, a transistor 4808, atransistor 4809, and a transistor 4810.

Note that the transistor 4805 corresponds to the transistor 1701 in FIG.17A, the transistor 4807 corresponds to the transistor 1702 in FIG. 17A,the transistor 4806 corresponds to the transistor 1703 in FIG. 17A, thetransistor 4808 corresponds to the transistor 1704 in FIG. 17A, thetransistor 4810 corresponds to the transistor 1705 in FIG. 17A, and thetransistor 4809 corresponds to the transistor 1706 in FIG. 17A. Inaddition, the transistor 4803 and the transistor 4804 correspond to thetransistor 1707 in FIG. 17A.

Connection relations of the flip-flop circuit in FIG. 48 are described.Note that a node of a second terminal of the transistor 4801, a secondterminal of the transistor 4800, a gate of the transistor 4806, a secondterminal of the transistor 4804, and a gate of the transistor 4802 isdenoted by N481. In addition, a node of a second terminal of thetransistor 4805, a second terminal of the transistor 4806, a secondterminal of the transistor 4807, and a gate of the transistor 4808 isdenoted by N482. Further, a node of a second terminal of the transistor4809, a second terminal of the transistor 4808, a second terminal of thetransistor 4810, a gate of the transistor 4803, and a gate of thetransistor 4804 is denoted by N483.

A gate of the transistor 4801 is connected to a wiring 4814, a firstterminal of the transistor 4801 is connected to a wiring 4811, and thesecond terminal of the transistor 4801 is connected to the node N481. Agate of the transistor 4800 is connected to a wiring 4815, a firstterminal of the transistor 4800 is connected to a wiring 4812, and thesecond terminal of the transistor 4800 is connected to the node N481.The gate of the transistor 4806 is connected to the node N481, a firstterminal of the transistor 4806 is connected to the wiring 4812, and thesecond terminal of the transistor 4806 is connected to the node N482. Agate of the transistor 4805 is connected to the wiring 4811, a firstterminal of the transistor 4805 is connected to the wiring 4811, and thesecond terminal of the transistor 4805 is connected to the node N482. Agate of the transistor 4807 is connected to a wiring 4813, a firstterminal of the transistor 4807 is connected to the wiring 4812, and thesecond terminal of the transistor 4807 is connected to the node N482.The gate of the transistor 4808 is connected to the node N482, a firstterminal of the transistor 4808 is connected to the wiring 4811, and thesecond terminal of the transistor 4808 is connected to the node N483. Agate of the transistor 4809 is connected to the node N481, a firstterminal of the transistor 4809 is connected to the wiring 4812, and thesecond terminal of the transistor 4809 is connected to the node N483. Agate of the transistor 4810 is connected to the wiring 4813, a firstterminal of the transistor 4810 is connected to the wiring 4812, and thesecond terminal of the transistor 4810 is connected to the node N483.The gate of the transistor 4804 is connected to the node N483, a firstterminal of the transistor 4804 is connected to the wiring 4812, and thesecond terminal of the transistor 4804 is connected to the node N481.The gate of the transistor 4803 is connected to the node N483, a firstterminal of the transistor 4803 is connected to the wiring 4812, and asecond terminal of the transistor 4803 is connected to a wiring 4816.The gate of the transistor 4802 is connected to the node N481, a firstterminal of the transistor 4802 is connected to the wiring 4813, and asecond terminal of the transistor 4802 is connected to the wiring 4816.

In addition, each of the transistors 4800 to 4810 is a P-channeltransistor.

Accordingly, since the flip-flop circuit in FIG. 48 can be formed byusing only P-channel transistors, a step of forming N-channeltransistors is not necessary. Thus, in the flip-flop circuit in FIG. 48,a manufacturing process can be simplified, so that manufacturing costcan be reduced and a yield can be improved.

In addition, the power supply potential VDD is supplied to the wiring4812 and the power supply potential VSS is supplied to the wiring 4811.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 4811 and the wiring 4812, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 4813, the wiring4814, and the wiring 4815. Note that the signal supplied to each of thewiring 4813, the wiring 4814, and the wiring 4815 is a binary digitalsignal. Note also that the power supply potential VDD, the power supplypotential VSS, or another power supply potential may be supplied to eachof the wiring 4813, the wiring 4814, and the wiring 4815. Alternatively,an analog signal may be supplied to each of the wiring 4813, the wiring4814, and the wiring 4815.

Next, operations of the flip-flop circuit shown in FIG. 48 are describedwith reference to FIG. 49.

FIG. 49 is an example of a timing chart of the flip-flop circuit shownin FIG. 48. The timing chart in FIG. 49 shows a potential of the wiring4813, a potential of the wiring 4814, a potential of the node N481, apotential of the node N482, a potential of the node N483, a potential ofthe wiring 4816, a relation of on/off of the transistor 4803 and thetransistor 4804, and a potential of the wiring 4815.

The timing chart in FIG. 48 is described by dividing the whole periodinto periods T1 to T4. In addition, the period T3 is described bydividing the whole period into a period T3 a and a period T3 b.

Note that the period T3 a and the period T4 are sequentially repeated inthe periods other than the periods T1, T2, and T3 b.

First, an operation in the period T1 is described. In the period T1, anH-level signal is supplied to the wiring 4813, an L-level signal issupplied to the wiring 4814, and an H-level signal is supplied to thewiring 4815.

Accordingly, the transistor 4801 is turned on, and the transistors 4800,4807, and 4810 are turned off. At this time, the power supply potentialVSS is supplied to the node N481 through the transistor 4801, so thatthe potential of the node N481 decreases. In addition, the transistor4806 and the transistor 4809 are turned on by the decrease in thepotential of the node N481, so that the potential of the node N482 andthe potential of the node N483 rise. Further, the transistor 4808 isturned off by the rise in the potential of the node N482. Moreover, thetransistor 4803 and the transistor 4804 are turned off by the rise inthe potential of the node N483.

Here, the decrease in the potential of the node N481 continues until thetransistor 4801 is turned off. The transistor 4801 is turned off whenthe potential of the node N481 becomes the sum of the power supplypotential VSS and the absolute value of a threshold voltage Vth4801 ofthe transistor 4801 (VSS+|Vth4801|). Therefore, the potential of thenode N481 becomes VSS+|Vth4801|, so that the node N481 becomes afloating state.

Therefore, the transistor 4802 is turned on. In addition, since theH-level signal of the wiring 4813 is supplied to the wiring 4816, thepotential of the wiring 4816 becomes equal to the power supply potentialVDD.

Next, an operation in the period T2 is described. In the period T2, anL-level signal is supplied to the wiring 4813, an H-level signal issupplied to the wiring 4814, and an H-level signal is supplied to thewiring 4815.

Accordingly, the transistor 4801 is turned off, the transistor 4800 iskept off, and the transistor 4807 and the transistor 4810 are turned on.At this time, the node N481 is in a floating state, and the potential ofthe node N481 is kept at VSS+|Vth4801|. In addition, the potential ofthe node N482 remains at an H level because the transistor 4806 and thetransistor 4807 are on. Further, the potential of the node N483 remainsat an H level because the transistor 4809 and the transistor 4810 areon. Thus, since the node N483 is at the H level, the transistor 4803 andthe transistor 4804 are kept off.

Here, the node N481 is in a floating state and kept at an L level. Inaddition, since the node N481 is kept at the L level, the transistor4802 is kept on. Further, since the L-level signal of the wiring 4813 issupplied to the wiring 4816, the potential of the wiring 4816 decreases.Therefore, the potential of the node N481 becomes equal to or lower thana value obtained by subtracting the absolute value of a thresholdvoltage Vth4802 of the transistor 4802 from the power supply potentialVSS (VSS−|Vth4802|) by a bootstrap operation, so that the potential ofthe wiring 4816 becomes equal to the power supply potential VSS.

Next, an operation in the period T3 b is described. In the period T3 b,an H-level signal is supplied to the wiring 4813, an H-level signal issupplied to the wiring 4814, and an L-level signal is supplied to thewiring 4815.

Accordingly, the transistor 4801 is kept off, the transistor 4800 isturned on, and the transistor 4807 and the transistor 4810 are turnedoff. At this time, the power supply potential VDD is supplied to thenode N481 through the transistor 4800, so that the potential of the nodeN481 rises. In addition, the transistor 4806 and the transistor 4807 areturned off by the rise in the potential of the node N481. Therefore, thepotential of the node N482 and the potential of the node N483 aredecreased by a bootstrap operation. The potential of the node N482decreases to a value equal to or lower than a value obtained bysubtracting the absolute value of a threshold voltage Vth4808 of thetransistor 4808 from the power supply potential VSS (VSS−|Vth4808|). Thepotential of the node N483 decreases to the power supply potential VSS.Therefore, the transistor 4803 and the transistor 4804 are turned on bythe decrease in the potential of the node N483.

In addition, the transistor 4802 is turned off by the rise in thepotential of the node N481. Therefore, since the power supply potentialVDD is supplied to the wiring 4816 through the transistor 4803, thepotential of the wiring 4816 becomes equal to the power supply potentialVDD.

Next, an operation in the period T4 is described. In the period T4, anL-level signal is supplied to the wiring 4813, an H-level signal issupplied to the wiring 4814, and an H-level signal is supplied to thewiring 4815.

Accordingly, the transistor 4801 is kept off, the transistor 4800 isturned off, and the transistor 4807 and the transistor 4810 are turnedon. At this time, the node N481 is in a floating state, and thepotential of the node N481 is kept at the power supply potential VDD.Thus, the transistor 4802, the transistor 4806, and the transistor 4809are kept off. In addition, the potential of the node N482 becomes an Hlevel because the power supply potential VDD is supplied thereto throughthe transistor 4807. Therefore, the transistor 4808 is turned off.Further, the potential of the node N483 becomes an H level because thepower supply potential VDD is supplied thereto through the transistor4810. Therefore, the transistor 4803 and the transistor 4804 are turnedoff.

Therefore, the wiring 4816 becomes a floating state, and the potentialof the wiring 4816 is kept equal to the power supply potential VDD.

Next, an operation in the period T3 a is described. In the period T3 a,an H-level signal is supplied to the wiring 4813, an H-level signal issupplied to the wiring 4814, and an H-level signal is supplied to thewiring 4815.

Accordingly, the transistor 4801 and the transistor 4800 are kept off,and the transistor 4807 and the transistor 4810 are turned off. At thistime, the node N481 is in a floating state, and the potential of thenode N481 is kept at the H level. Thus, the transistor 4802, thetransistor 4806, and the transistor 4809 are kept off. Therefore, thepotential of the node N482 and the potential of the node N483 aredecreased by a bootstrap operation. The potential of the node N482decreases to be equal to or lower than the value obtained by subtractingthe absolute value of the threshold voltage Vth4808 of the transistor4808 from the power supply potential VSS (VSS−|Vth4808|). The potentialof the node N483 decreases to the power supply potential VSS. Therefore,the transistor 4803 and the transistor 4804 are turned on by thedecrease in the potential of the node N483.

Further, since the power supply potential VDD is supplied to the wiring4816 through the transistor 4803, the potential of the wiring 4816 iskept equal to the power supply potential VDD.

By the above-described operations, the flip-flop circuit in FIG. 48keeps the node N481 at an L level to be in a floating state in theperiod T1. In the period T2, the flip-flop circuit in FIG. 48 sets thepotential of the node N481 equal to or lower than VSS-|Vth4802| by thebootstrap operation, so that the potential of the wiring 4816 is madeequal to the power supply potential VSS.

Further, in the period T3 a, the flip-flop circuit in FIG. 48 turns onthe transistor 4803 and the transistor 4804, and supplies the powersupply potential VDD to the wiring 4816 and the node N481. In the periodT4, the flip-flop circuit in FIG. 48 turns off the transistor 4803 andthe transistor 4804. Therefore, since the flip-flop circuit in FIG. 48sequentially turns on the transistor 4803 and the transistor 4804, itcan suppress characteristic deterioration of the transistor 4803 and thetransistor 4804, so that the potential of each of the node N481 and thewiring 4816 can be stably kept equal to the power supply potential VDD.

In addition, the flip-flop circuit in FIG. 48 can set the potential ofthe node N483 equal to the power supply potential VSS in the periods T3a and T3 b. Therefore, even when characteristics of the transistor 4803and the transistor 4804 deteriorate, the flip-flop circuit in FIG. 48can be operated under a wide range of operating conditions.

In addition, the flip-flop circuit in FIG. 48 does not include atransistor which is on in all of the periods T1 to T4. That is, theflip-flop circuit in FIG. 48 does not include a transistor which isalways or almost always on. Accordingly, the flip-flop circuit in FIG.48 can suppress characteristic deterioration of a transistor and athreshold voltage shift due to the characteristic deterioration.

Note that the transistors 4801 to 4810 have functions which are similarto those of the transistors 3601 to 3610.

Note that a two-input NAND circuit in which the node N481 and the wiring4813 correspond to an input terminal and the node N483 corresponds to anoutput terminal is constructed from the transistors 4805 to 4810.

Note that as shown in FIG. 50, a capacitor 5001 may be provided betweenthe gate (the node N482) and the second terminal (the node N483) of thetransistor 4808. This is because the potential of the node N482 and thepotential of the node N483 are decreased by the bootstrap operation inthe periods T3 a and T3 b, so that the flip-flop circuit can easilyperform the bootstrap operation by proving the capacitor 5001.

Note that as shown in FIG. 51, the transistor 4807 is not necessarilyprovided.

Note that as shown in FIG. 52, a capacitor 5201 may be provided betweenthe gate (the node N481) and the second terminal (the wiring 4816) ofthe transistor 4802. This is because the potential of the node N481 andthe potential of the wiring 4816 are raised by the bootstrap operationin the period T2, so that the flip-flop circuit can easily perform thebootstrap operation by proving the capacitor 5201.

Note that it is only necessary that the transistor 4801 make the nodeN481 into a floating state in the period T1 so that the potential of thenode N481 becomes an L level. Therefore, even when the first terminal ofthe transistor 4801 is connected to the wiring 4814, the transistor 4801can set the node N481 into a floating state so that the potential of thenode N481 becomes an L level.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 7

In this embodiment mode, the case is described in which the basiccircuit described in Embodiment Mode 4 is applied to a flip-flopcircuit, with reference to FIG. 56.

FIG. 56 is an example of a flip-flop circuit to which the basic circuitin FIG. 25A described in Embodiment Mode 4 is applied. The flip-flopcircuit in FIG. 56 includes a transistor 5601, a transistor 5602, atransistor 5603, a transistor 5604, a transistor 5605, a transistor5606, a transistor 5607, a transistor 5608, a circuit 5608, and acircuit 5609.

Note that as the circuit 5608 and the circuit 5609, the NOR circuit 2715in FIG. 27 and the NOR circuit 3617 in FIG. 36 can be used.

Connection relations of the flip-flop circuit in FIG. 56 are described.Note that a node of a second terminal of the transistor 5601, a secondterminal of the transistor 5607, a second terminal of the transistor5605, a second terminal of the transistor 5606, and a gate of thetransistor 5602 is denoted by N561. In addition, a node of a gate of thetransistor 5604 and a gate of the transistor 5606 is denoted by N562.Further, a node of a gate of the transistor 5603 and a gate of thetransistor 5605 is denoted by N563.

A gale of the transistor 5601 is connected to a wiring 5614, a firstterminal of the transistor 5601 is connected to a wiring 5610, and thesecond terminal of the transistor 5601 is connected to the node N561. Agate of the transistor 5607 is connected to a wiring 5615, a firstterminal of the transistor 5607 is connected to a wiring 5611, and thesecond terminal of the transistor 5607 is connected to the node N561.Two input terminals of the circuit 5608 are connected to the node N561and a wiring 5612, respectively, and an output terminal of the circuit5608 is connected to the node N562. Two input terminals of the circuit5609 are connected to the node N561 and a wiring 5613, respectively, andan output terminal of the circuit 5609 is connected to the node N563.The gate of the transistor 5606 is connected to the node N562, a firstterminal of the transistor 5606 is connected to the wiring 5611, and thesecond terminal of the transistor 5606 is connected to the node N561.The gate of the transistor 5605 is connected to the node N563, a firstterminal of the transistor 5605 is connected to the wiring 5611, and thesecond terminal of the transistor 5605 is connected to the node N561.The gate of the transistor 5604 is connected to the node N562, a firstterminal of the transistor 5604 is connected to the wiring 5611, and asecond terminal of the transistor 5604 is connected to a wiring 5616.The gate of the transistor 5603 is connected to the node N563, a firstterminal of the transistor 5603 is connected to the wiring 5611, and asecond terminal of the transistor 5603 is connected to the wiring 5616.The gate of the transistor 5602 is connected to the node N561, a firstterminal of the transistor 5602 is connected to the wiring 5613, and asecond terminal of the transistor 5602 is connected to the wiring 5616.

In addition, each of the transistors 5601 to 5607 is an N-channeltransistor. Each of transistors included in the circuit 5608 and thecircuit 5609 is also an N-channel transistor.

Accordingly, since the flip-flop circuit in FIG. 56 can be formed byusing only N-channel transistors, amorphous silicon can be used for asemiconductor layer of the flip-flop circuit in FIG. 56. Thus, amanufacturing process can be simplified, so that manufacturing cost canbe reduced and a yield can be improved. In addition, a semiconductordevice such as a large display panel can also be formed. Further, whenpolysilicon or single crystalline silicon is used for the semiconductorlayer of the flip-flop circuit in FIG. 56, the manufacturing process canbe simplified.

In addition, the power supply potential VDD is supplied to the wiring5610 and the power supply potential VSS is supplied to the wiring 5611.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 5610 and the wiring 5611, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wiring 5612, the wiring5613, the wiring 5614, and the wiring 5615. Note that the signalsupplied to each of the wiring 5612, the wiring 5613, the wiring 5614,and the wiring 5615 is a binary digital signal. Note also that the powersupply potential VDD, the power supply potential VSS, or another powersupply potential may be supplied to each of the wiring 5612, the wiring5613, the wiring 5614, and the wiring 5615. Alternatively, an analogsignal may be supplied to each of the wiring 5612, the wiring 5613, thewiring 5614, and the wiring 5615.

Next, operations of the flip-flop circuit shown in FIG. 56 are describedwith reference to FIG. 57.

FIG. 57 is an example of a timing chart of the flip-flop circuit shownin FIG. 56. The timing chart in FIG. 57 shows a potential of the wiring5612, a potential of the wiring 5613, a potential of the wiring 5614, apotential of the node N561, a potential of the node N562, a potential ofthe node N563, a potential of the wiring 5616, a relation of on/off ofthe transistor 5604 and the transistor 5606, a relation of on/off of thetransistor 5603 and the transistor 5605, and a potential of the wiring5615.

The timing chart in FIG. 57 is described by dividing the whole periodinto periods T1 to T4. In addition, the period T3 is described bydividing the whole period into a period T3 a and a period T3 b.

Note that the period T3 a and the period T4 are sequentially repeated inthe periods other than the periods T1, T2, and T3 b.

First, an operation in the period T1 is described. In the period T1, anH-level signal is supplied to the wiring 5612, an L-level signal issupplied to the wiring 5613, an H-level signal is supplied to the wiring5614, and an L-level signal is supplied to the wiring 5615.

Accordingly, the transistor 5601 is turned on and the transistor 5607 isturned off. At this time, the power supply potential VDD is supplied tothe node N561 through the transistor 5601, so that the potential of thenode N561 rises. Therefore, the circuit 5608 outputs an L-level signalto the node N562, and the transistor 5604 and the transistor 5606 areturned off. In addition, the circuit 5609 outputs an L-level signal tothe node N563, and the transistor 5603 and the transistor 5605 areturned off.

Note that rise in the potential of the node N561 continues until thetransistor 5601 is turned off. The transistor 5601 is turned off whenthe potential of the node N561 becomes a value obtained by subtracting athreshold voltage Vth5601 of the transistor 5601 from the power supplypotential VDD (VDD−Vth5601). Therefore, the potential of the node N561becomes VDD-Vth5601, and the node N561 becomes a floating state.

Therefore, the transistor 5602 is turned on. Since the L-level signal ofthe wiring 5613 is supplied to the wiring 5616 through the transistor5602, the potential of the wiring 5616 becomes equal to the power supplypotential VSS.

Next, an operation in the period T2 is described. In the period T2, anL-level signal is supplied to the wiring 5612, an H-level signal issupplied to the wiring 5613, an L-level signal is supplied to the wiring5614, and an L-level signal is supplied to the wiring 5615.

Accordingly, the transistor 5601 is turned off and the transistor 5607is kept off. At this time, the node N561 is kept at VDD−Vth5601. Thus,the circuit 5608 outputs an L-level signal to the node N562, and thetransistor 5604 and the transistor 5606 are kept off. In addition, thecircuit 5609 outputs an L-level signal to the node N563, and thetransistor 5603 and the transistor 5605 are kept off.

Note that since an H-level signal is supplied to the wiring 5613, thepotential of the wiring 5616 starts to rise. Therefore, the potential ofthe node N561 becomes equal to or higher than the sum of the powersupply potential VDD and a threshold voltage Vth5602 of the transistor5602 (VDD+Vth5602) by a bootstrap operation. Thus, the potential of thewiring 5616 rises to be equal to the power supply potential VDD.

Next, an operation in the period T3 b is described. In the period T3 b,an H-level signal is supplied to the wiring 5612, an L-level signal issupplied to the wiring 5613, an L-level signal is supplied to the wiring5614, and an H-level signal is supplied to the wiring 5615.

Accordingly, the transistor 5601 is turned off and the transistor 5607is turned on. Since the power supply potential VSS is supplied to thenode N561 through the transistor 5607, the potential of the node N561decreases. Thus, the circuit 5608 outputs an L-level signal to the nodeN562, and the transistor 5604 and the transistor 5606 are kept off. Inaddition, the circuit 5609 outputs an H-level signal to the node N563,and the transistor 5603 and the transistor 5605 are turned on.

Note that since the node N561 becomes an L level, the transistor 5602 isturned off. Since the power supply potential VSS is supplied to thewiring 5616 through the transistor 5603, the potential of the wiring5616 is kept equal to the power supply potential VSS.

Next, an operation in the period T4 is described. In the period T4, anL-level signal is supplied to the wiring 5612, an H-level signal issupplied to the wiring 5613, an L-level signal is supplied to the wiring5614, and an L-level signal is supplied to the wiring 5615.

Accordingly, the transistor 5601 is kept off and the transistor 5607 isturned off. The potential of the node N561 is kept at the L level. Thus,the circuit 5608 outputs an H-level signal to the node N562, and thetransistor 5604 and the transistor 5606 are turned on. In addition, thecircuit 5609 outputs an L-level signal to the node N563, and thetransistor 5603 and the transistor 5605 are turned off.

Note that since the node N561 is kept at the L level, the transistor5602 is turned off. Since the power supply potential VSS is supplied tothe wiring 5616 through the transistor 5604, the potential of the wiring5616 is kept equal to the power supply potential VSS.

Next, an operation in the period T3 a is described. In the period T3 a,an H-level signal is supplied to the wiring 5612, an L-level signal issupplied to the wiring 5613, an L-level signal is supplied to the wiring5614, and an 11-level signal is supplied to the wiring 5615.

Accordingly, the transistor 5601 is turned off and the transistor 5607is turned on. The potential of the node N561 is kept at the L level.Thus, the circuit 5608 outputs an L-level signal to the node N562, andthe transistor 5604 and the transistor 5606 are turned off. In addition,the circuit 5609 outputs an H-level signal to the node N563, and thetransistor 5603 and the transistor 5605 are turned on.

Note that since the node N561 is kept at the L level, the transistor5602 is turned off. Since the power supply potential VSS is supplied tothe wiring 5616 through the transistor 5603, the potential of the wiring5616 is kept equal to the power supply potential VSS.

By the above-described operations, the flip-flop circuit in FIG. 56keeps the node N561 at an H level to be in a floating state in theperiod T1. In the period T2, the flip-flop circuit in FIG. 56 sets thepotential of the node N561 equal to or higher than VDD+Vth5602 by thebootstrap operation, so that the potential of the wiring 5616 is madeequal to the power supply potential VDD.

In addition, the transistor 5603 is turned on, and the power supplypotential VSS is supplied to the wiring 5616 in the period T3 a.Further, the transistor 5604 is turned on, and the power supplypotential VSS is supplied to the wiring 5616 in the period T4.Therefore, the flip-flop circuit in FIG. 56 can always supply the powersupply potential VSS to the wiring 5616 in the periods T3 a and T4.

In the period T3 b, the transistor 5605 is turned on and the powersupply potential VSS is supplied to the node N561. Further, in theperiod T4, the transistor 5606 is turned on, and the power supplypotential VSS is supplied to the node N561. Therefore, the flip-flopcircuit in FIG. 56 can always supply the power supply potential VSS tothe node N561 in the periods T3 b and T4.

In addition, the flip-flop circuit in FIG. 56 does not include atransistor which is on in all of the periods T1 to T4. That is, theflip-flop circuit in FIG. 56 does not include a transistor which isalways or almost always on. Accordingly, the flip-flop circuit in FIG.56 can suppress characteristic deterioration of a transistor and athreshold voltage shift due to the characteristic deterioration.

Further, the characteristics of a transistor which is formed ofamorphous silicon easily deteriorate. Therefore, when the transistorincluded in the flip-flop circuit in FIG. 56 is formed using amorphoussilicon, not only can the advantages such as a reduction inmanufacturing cost and improvement in a yield be obtained, but also theproblem of the characteristic deterioration of the transistor can besolved.

Here, the functions of the transistors 5601 to 5607 are described. Thetransistor 5601 has a function of a switch which selects whether toconnect the wiring 5610 and the node N561 in accordance with thepotential of the wiring 5614. The transistor 5602 has a function of aswitch which selects whether to connect the wiring 5613 and the wiring5616 in accordance with the potential of the node N561. The transistor5603 has a function of a switch which selects whether to connect thewiring 5611 and the wiring 5616 in accordance with the potential of thenode N563. The transistor 5604 has a function of a switch which selectswhether to connect the wiring 5611 and the wiring 5616 in accordancewith the potential of the node N562. The transistor 5605 has a functionof a switch which selects whether to connect the wiring 5611 and thenode N561 in accordance with the potential of the node N563. Thetransistor 5606 has a function of a switch which selects whether toconnect the wiring 5611 and the node N561 in accordance with thepotential of the node N562. The transistor 5607 has a function of aswitch which selects whether to connect the wiring 5611 and the nodeN561 in accordance with the potential of the wiring 5615.

Next, the case is described in which the flip-flop circuit shown in FIG.56 is constructed from P-channel transistors, with reference to FIG. 58.

FIG. 58 is an example of a flip-flop circuit to which the basic circuitin FIG. 26A described in Embodiment Mode 4 is applied. The flip-flopcircuit in FIG. 58 includes a transistor 5801, a transistor 5802, atransistor 5803, a transistor 5804, a transistor 5805, a transistor5806, a transistor 5807, a circuit 5808, and a circuit 5809.

Note that as the circuit 5808 and the circuit 5809, the NAND circuit4415 in FIG. 44 and the NAND circuit 4817 in FIG. 48 can be used.

Connection relations of the flip-flop circuit in FIG. 58 are described.Note that a node of a second terminal of the transistor 5801, a secondterminal of the transistor 5807, a second terminal of the transistor5805, a second terminal of the transistor 5806, and a gate of thetransistor 5802 is denoted by N581. In addition, a node of a gate of thetransistor 5804 and a gate of the transistor 5806 is denoted by N582.Further, a node of a gate of the transistor 5803 and a gate of thetransistor 5805 is denoted by N563.

A gate of the transistor 5801 is connected to a wiring 5814, a firstterminal of the transistor 5801 is connected to a wiring 5810, and thesecond terminal of the transistor 5801 is connected to the node N581. Agate of the transistor 5807 is connected to a wiring 5815, a firstterminal of the transistor 5807 is connected to a wiring 5811, and thesecond terminal of the transistor 5807 is connected to the node N581.Two input terminals of the circuit 5808 are connected to the node N581and a wiring 5812, respectively, and an output terminal of the circuit5808 is connected to the node N582. Two input terminals of the circuit5809 are connected to the node N581 and a wiring 5813, respectively, andan output terminal of the circuit 5809 is connected to the node N583.The gate of the transistor 5806 is connected to the node N582, a firstterminal of the transistor 5806 is connected to the wiring 5811, and thesecond terminal of the transistor 5806 is connected to the node N581.The gate of the transistor 5805 is connected to the node N583, a firstterminal of the transistor 5805 is connected to the wiring 5811, and thesecond terminal of the transistor 5805 is connected to the node N581.The gate of the transistor 5804 is connected to the node N582, a firstterminal of the transistor 5804 is connected to the wiring 5811, and asecond terminal of the transistor 5804 is connected to a wiring 5816.The gate of the transistor 5803 is connected to the node N583, a firstterminal of the transistor 5803 is connected to the wiring 5811, and asecond terminal of the transistor 5803 is connected to the wiring 5816.The gate of the transistor 5802 is connected to the node N581, a firstterminal of the transistor 5802 is connected to the wiring 5813, and asecond terminal of the transistor 5802 is connected to the wiring 5816.

In addition, each of the transistors 5801 to 5807 is a P-channeltransistor. Each of transistors included in the circuit 5808 and thecircuit 5809 is also a P-channel transistor.

Accordingly, since the flip-flop circuit in FIG. 58 can be formed byusing only P-channel transistors, a step of forming N-channeltransistors is not necessary. Thus, in the flip-flop circuit in FIG. 58,a manufacturing process can be simplified, so that manufacturing costcan be reduced and a yield can be improved.

In addition, the power supply potential VDD is supplied to the wiring5811 and the power supply potential VSS is supplied to the wiring 5810.Note that the power supply potential VDD is higher than the power supplypotential VSS. Note also that a digital signal, an analog signal, or thelike may be supplied to each of the wiring 5810 and the wiring 5811, oranother power supply potential may be supplied thereto.

In addition, a signal is supplied to each of the wirings 5812 to 5815.Note that the signal supplied to each of the wirings 5812 to 5815 is abinary digital signal. Note also that the power supply potential VDD,the power supply potential VSS, or another power supply potential may besupplied to each of the wirings 5812 to 5815. Alternatively, an analogsignal may be supplied to each of the wirings 5812 to 5815.

Next, operations of the flip-flop circuit shown in FIG. 58 are describedwith reference to FIG. 59.

FIG. 59 is an example of a timing chart of the flip-flop circuit shownin FIG. 58. The timing chart in FIG. 59 shows a potential of the wiring5812, a potential of the wiring 5813, a potential of the wiring 5814, apotential of the node N581, a potential of the node N582, a potential ofthe node N583, a potential of the wiring 5816, a relation of on/off ofthe transistor 5804 and the transistor 5806, a relation of on/off of thetransistor 5803 and the transistor 5805, and a potential of the wiring5815.

The timing chart in FIG. 59 is described by dividing the whole periodinto periods T1 to T4. In addition, the period T3 is described bydividing the whole period into a period T3 a and a period T3 b.

Note that the period T3 a and the period T4 are sequentially repeated inthe periods other than the periods T1, T2, and T3 b.

First, an operation in the period T1 is described. In the period T1, anL-level signal is supplied to the wiring 5812, an H-level signal issupplied to the wiring 5813, an L-level signal is supplied to the wiring5814, and an H-level signal is supplied to the wiring 5815.

Accordingly, the transistor 5801 is turned on and the transistor 5807 isturned off. At this time, the power supply potential VSS is supplied tothe node N581 through the transistor 5801, so that the potential of thenode N581 decreases. Therefore, the circuit 5808 outputs an H-levelsignal to the node N582, and the transistor 5804 and the transistor 5806are turned off. In addition, the circuit 5809 outputs an H-level signalto the node N583, and the transistor 5803 and the transistor 5805 areturned off.

Note that decrease in the potential of the node N581 continues until thetransistor 5801 is turned off. The transistor 5801 is turned off whenthe potential of the node N581 becomes equal to the sum of the powersupply potential VSS and the absolute value of a threshold voltageVth5801 of the transistor 5801 (VSS+|Vth5801|). Therefore, the potentialof the node N581 becomes VSS+|Vth5801|, and the node N581 becomes afloating state.

Therefore, the transistor 5802 is turned on. Since the H-level signal ofthe wiring 5813 is supplied to the wiring 5816 through the transistor5802, the potential of the wiring 5816 becomes equal to the power supplypotential VDD.

Next, an operation in the period T2 is described. In the period T2, anH-level signal is supplied to the wiring 5812, an L-level signal issupplied to the wiring 5813, an H-level signal is supplied to the wiring5814, and an H-level signal is supplied to the wiring 5815.

Accordingly, the transistor 5801 is turned off and the transistor 5807is kept off. At this time, the potential of the node N581 is kept atVSS+|Vth5801|. Thus, the circuit 5808 outputs an H-level signal to thenode N582, and the transistor 5804 and the transistor 5806 are kept off.In addition, the circuit 5809 outputs an H-level signal to the nodeN583, and the transistor 5803 and the transistor 5805 are kept off.

Note that since an L-level signal is supplied to the wiring 5813, thepotential of the wiring 5816 starts to decrease. Therefore, thepotential of the node N581 becomes equal to or lower than a valueobtained by subtracting the absolute value of a threshold voltageVth5802 of the transistor 5802 from the power supply potential VSS(VSS−|Vth5802|) by a bootstrap operation. Thus, the potential of thewiring 5816 decreases to be equal to the power supply potential VSS.

Next, an operation in the period T3 b is described. In the period T3 b,an L-level signal is supplied to the wiring 5812, an H-level signal issupplied to the wiring 5813, an H-level signal is supplied to the wiring5814, and an L-level signal is supplied to the wiring 5815.

Accordingly, the transistor 5801 is turned off and the transistor 5807is turned on. Since the power supply potential VDD is supplied to thenode N581 through the transistor 5807, the potential of the node N561rises. Thus, the circuit 5808 outputs an H-level signal to the nodeN582, and the transistor 5804 and the transistor 5806 are kept off. Inaddition, the circuit 5809 outputs an L-level signal to the node N583,and the transistor 5803 and the transistor 5805 are turned on.

Note that since the node N581 becomes an H level, the transistor 5802 isturned off. Since the power supply potential VDD is supplied to thewiring 5816 through the transistor 5803, the potential of the wiring5816 becomes equal to the power supply potential VDD.

Next, an operation in the period T4 is described. In the period T4, anH-level signal is supplied to the wiring 5812, an L-level signal issupplied to the wiring 5813, an H-level signal is supplied to the wiring5814, and an H-level signal is supplied to the wiring 5815.

Accordingly, the transistor 5801 is kept off and the transistor 5807 isturned off. The potential of the node N581 is kept at the H level. Thus,the circuit 5808 outputs an L-level signal to the node N582, and thetransistor 5804 and the transistor 5806 are turned on. In addition, thecircuit 5809 outputs an H-level signal to the node N583, and thetransistor 5803 and the transistor 5805 are turned off.

Note that since the node N581 is kept at the H level, the transistor5802 is turned off. Since the power supply potential VDD is supplied tothe wiring 5816 through the transistor 5804, the potential of the wiring5816 is kept equal to the power supply potential VDD.

Next, an operation in the period T3 a is described. In the period T3 a,an L-level signal is supplied to the wiring 5812, an H-level signal issupplied to the wiring 5813, an H-level signal is supplied to the wiring5814, and an H-level signal is supplied to the wiring 5815.

Accordingly, the transistor 5801 is turned off and the transistor 5807is turned off. The potential of the node N581 is kept at the H level.Thus, the circuit 5808 outputs an H-level signal to the node N582, andthe transistor 5804 and the transistor 5806 are turned off. In addition,the circuit 5809 outputs an L-level signal to the node N583, and thetransistor 5803 and the transistor 5805 are turned on.

Note that since the node N581 is kept at the H level, the transistor5802 is turned off. Since the power supply potential VDD is supplied tothe wiring 5816 through the transistor 5803, the potential of the wiring5816 is kept equal to the power supply potential VDD.

By the above-described operations, the flip-flop circuit in FIG. 58keeps the node N581 at an L level to be in a floating state in theperiod T1. In the period T2, the flip-flop circuit in FIG. 58 sets thepotential of the node N581 equal to or lower than VSS−|Vth5802| by thebootstrap operation, so that the potential of the wiring 5816 is madeequal to the power supply potential VSS.

In addition, the transistor 5803 is turned on, and the power supplypotential VDD is supplied to the wiring 5816 in the period T3 a.Further, the transistor 5804 is turned on, and the power supplypotential VDD is supplied to the wiring 5816 in the period T4.Therefore, the flip-flop circuit in FIG. 58 can always supply the powersupply potential VDD to the wiring 5816 in the periods T3 a and T4.

In addition, the transistor 5805 is turned on, and the power supplypotential VDD is supplied to the node N581 in the period T3 b. Further,the transistor 5806 is turned on, and the power supply potential VDD issupplied to the node N581 in the period T4. Therefore, the flip-flopcircuit in FIG. 58 can always supply the power supply potential VDD tothe node N581 in the periods T3 b and T4.

In addition, the flip-flop circuit in FIG. 58 does not include atransistor which is on in all of the periods T1 to T4. That is, theflip-flop circuit in FIG. 58 does not include a transistor which isalways or almost always on. Accordingly, the flip-flop circuit in FIG.58 can suppress characteristic deterioration of a transistor and athreshold voltage shift due to the characteristic deterioration.

Note that the transistors 5801 to 5807 have functions which are similarto those of the transistors 5601 to 5607.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 8

This embodiment mode will describe a shift register which employs theflip-flop circuits described in Embodiment Modes 5 and 6, with referenceto FIG. 60.

FIG. 60 shows an example of a shift register which employs the flip-flopcircuits described in Embodiment Modes 5 and 6. The shift register inFIG. 60 includes a plurality of flip-flop circuits 6001.

Note that the flip-flop circuits 6001 are similar to those shown inEmbodiment Modes 5 and 6.

In FIG. 60, a flip-flop circuit 6001(n−1) of an (n−1)th stage, aflip-flop circuit 6001(n) of an n-th stage, and a flip-flop circuit6001(n+1) of an (n+1)th stage are shown. Note that n is an even number.Note also that input terminals IN601 of the flip-flop circuits in theeven-numbered stages are connected to a wiring 6005, and input terminalsIN601 of the flip-flop circuits in the odd-numbered stages are connectedto a wiring 6004.

Note that the input terminals IN 601 are connected to each of the wiring2711 in FIG. 27, the wiring 3613 in FIG. 36, the wiring 4411 in FIG. 44,and the wiring 4813 in FIG. 48. Input terminals IN 602 are connected toeach of the wiring 2712 in FIG. 27, the wiring 3614 in FIG. 36, thewiring 4412 in FIG. 44, and the wiring 4814 in FIG. 48. Input terminalsIN 603 are connected to each of the wiring 2713 in FIG. 27, the wiring3615 in FIG. 36, the wiring 4413 in FIG. 44, and the wiring 4815 in FIG.48. Input terminals IN 604 are connected to each of the wiring 2709 inFIG. 27, the wiring 3611 in FIG. 36, the wiring 4410 in FIG. 44, and thewiring 4812 in FIG. 48. Input terminals IN 605 are connected to each ofthe wiring 2710 in FIG. 27, the wiring 3612 in FIG. 36, the wiring 4409in FIG. 44, and the wiring 4812 in FIG. 48. Output terminals IN 606 areconnected to each of the wiring 2714 in FIG. 27, the wiring 3616 in FIG.36, the wiring 4414 in FIG. 44, and the wiring 4816 in FIG. 48.

The power supply potential VDD is supplied to a wiring 6002, and thepower supply potential VSS is supplied to a wiring 6003. Note that thepower supply potential VDD is higher than the power supply potentialVSS. However, digital signals, analog signals, other power supplypotentials or the like may be supplied to the wiring 6002 and the wiring6003.

Signals are supplied to the wiring 6004, the wiring 6005, and a wiring6006. Note that the signal supplied to each of the wiring 6004, thewiring 6005, and the wiring 6006 is a binary digital signal. However,the power supply potential VDD, the power supply potential VSS, oranother power supply potential may be supplied to each of the wiring6004, the wiring 6005, and the wiring 6006. Alternatively, an analogsignal may be supplied to each of the wiring 6004, the wiring 6005, andthe wiring 6006.

Note that an output signal of the flip-flop circuit 6001 of an (n−2)thstage is supplied to the wiring 6006.

Next, an operation of the shift register shown in FIG. 60 will bedescribed with reference to a timing chart in FIG. 61.

FIG. 61 shows an example of a timing chart of the shift register shownin FIG. 60. The timing chart in FIG. 61 shows a potential of the wiring6004, a potential of the wiring 6005, a potential of an output terminalOUT606(n−2), a potential of an output terminal OUT606(n−1), a potentialof an output terminal OUT606(n), and a potential of an output terminalOUT606(n+1).

Note that the timing chart in FIG. 61 shows the case where the flip-flopcircuits 6001 are constructed from N-channel transistors. When theflip-flop circuits 6001 are constructed from P-channel transistors, itis only necessary to invert H-level signals and L-level signals.

Note that the timing chart in FIG. 61 will be described by dividing thewhole period into a period T1 to a period T8.

First, an operation in the period T1 is described. In the period T1, theflip-flop circuit 6001(n−1) performs the operation in the period T1shown in Embodiment Modes 5 and 6; the flip-flop circuit 6001(n)performs the operation in the period T4 shown in Embodiment Modes 5 and6; and the flip-flop circuit 6001(n+1) performs the operation in theperiod T3 a shown in Embodiment Modes 5 and 6.

Next, an operation in the period T2 is described. In the period T2, theflip-flop circuit 6001(n−1) performs the operation in the period T2shown in Embodiment Modes 5 and 6; the flip-flop circuit 6001(n)performs the operation in the period T1 shown in Embodiment Modes 5 and6; and the flip-flop circuit 6001(n+1) performs the operation in theperiod T4 shown in Embodiment Modes 5 and 6.

Therefore, an H-level signal is output from the output terminal OUT606of the flip-flop circuit 6001(n−1).

Next, an operation in the period T3 is described. In the period T3, theflip-flop circuit 6001(n−1) performs the operation in the period T3 bshown in Embodiment Modes 5 and 6; the flip-flop circuit 6001(n)performs the operation in the period T2 shown in Embodiment Modes 5 and6; and the flip-flop circuit 6001(n+1) performs the operation in theperiod T1 shown in Embodiment Modes 5 and 6.

Therefore, an H-level signal is output from the output terminal OUT606of the flip-flop circuit 6001(n).

Next, an operation in the period T4 is described. In the period T4, theflip-flop circuit 6001(n−1) performs the operation in the period T4shown in Embodiment Modes 5 and 6; the flip-flop circuit 6001(n)performs the operation in the period T3 b shown in Embodiment Modes 5and 6; and the flip-flop circuit 6001(n+1) performs the operation in theperiod T2 shown in Embodiment Modes 5 and 6.

Therefore, an H-level signal is output from the output terminal OUT606of the flip-flop circuit 6001(n+1).

Next, an operation in the period T5 is described. In the period T5, theflip-flop circuit 6001(n−1) performs the operation in the period T3 ashown in Embodiment Modes 5 and 6; the flip-flop circuit 6001(n)performs the operation in the period T4 shown in Embodiment Modes 5 and6; and the flip-flop circuit 6001(n+1) performs the operation in theperiod T3 b shown in Embodiment Modes 5 and 6.

Next, an operation in the period T6 is described. In the period T6, theflip-flop circuit 6001(n−1) performs the operation in the period T4shown in Embodiment Modes 5 and 6; the flip-flop circuit 6001(n)performs the operation in the period T3 a shown in Embodiment Modes 5and 6; and the flip-flop circuit 6001(n+1) performs the operation in theperiod T4 shown in Embodiment Modes 5 and 6.

Next, an operation in the period T7 is described. In the period T7, theflip-flop circuit 6001(n−1) performs the operation in the period T3 ashown in Embodiment Modes 5 and 6; the flip-flop circuit 6001(n)performs the operation in the period T4 shown in Embodiment Modes 5 and6; and the flip-flop circuit 6001(n+1) performs the operation in theperiod T3 a shown in Embodiment Modes 5 and 6.

Next, an operation in the period T8 is described. In the period T8, theflip-flop circuit 6001(n−1) performs the operation in the period T4shown in Embodiment Modes 5 and 6; the flip-flop circuit 6001(n)performs the operation in the period T3 a shown in Embodiment Modes 5and 6; and the flip-flop circuit 6001(n+1) performs the operation in theperiod T4 shown in Embodiment Modes 5 and 6.

In this manner, when the flip-flop circuits shown in Embodiment Modes 5and 6 are used for the shift register shown in FIG. 60, all of thetransistors included in the shift register can be either N-channel typeor P-channel type.

In addition, since all of the transistors included in the shift registershown in FIG. 60 can be N-channel transistors, amorphous silicon can beused for a semiconductor layer, which leads to a simplifiedmanufacturing process. Therefore, reduction in manufacturing cost andimprovement in a yield can be achieved. Further, a large display panelcan be formed. In addition, when the shift register shown in FIG. 60 isused for a semiconductor device, the semiconductor device can have along operating life even when amorphous silicon whose characteristicswill easily deteriorate is used.

The characteristics of a transistor which is formed of amorphous siliconeasily deteriorate. Therefore, when the transistors included in theshift register in FIG. 60 are formed using amorphous silicon, not onlycan the advantages such as a reduction in manufacturing cost andimprovement in a yield be obtained, but also the problem of thecharacteristic deterioration of the transistors can be solved.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 9

This embodiment mode will describe a source driver which employs theshift register described in Embodiment Mode 8, with reference to FIG.62.

A circuit shown in FIG. 62 is an example of a circuit configurationwhich employs the shift register shown in Embodiment Mode 8.

The circuit shown in FIG. 62 includes a shift register 6501 and aplurality of switches 6503. In addition, the shift register 6501 has aplurality of output terminals OUT.

In FIG. 62, switches 6503, loads 6504, and the output terminals OUT of afirst stage, a second stage, a third stage, and an n-th stage are shown.In addition, n is a natural number not less than two.

The shift register 6501 is similar to that shown in Embodiment Mode 8.

As shown in the circuit in FIG. 62, a wiring 6502 is connected to theloads 6504 through the switches 6503. In addition, the switches 6503 arecontrolled by the shift register 6501.

In addition, a transmission signal is supplied to the wiring 6502. Thetransmission signal may be either current or voltage.

Note that a plurality of control signals and various power supplypotentials are supplied to the shift register 6501, though not shown.

Next, an operation of the circuit shown in FIG. 62 is described.

The shift register 6501 sequentially outputs H-level signals or L-levelsignals from an output terminal OUT (1) of the first stage. At the sametime, the switches 6503 are sequentially turned on from the first stage.Then, transmission signals are sequentially supplied to the loads 6504through the switches 6503 from the first stage.

Note that when H-level signals are sequentially output from the outputterminal OUT (1) of the first stage, N-channel transistors are used asthe switches 6503. On the other hand, when L-level signals aresequentially output from the output terminal OUT (1) of the first stage,P-channel transistors are used as the switches 6503.

In the circuit in FIG. 62, when transmission signals are changed aton/off timing of the switches 6503, different voltages or currents canbe supplied to the plurality of loads 6504.

Here, the functions of the shift register 6501 and the switches 6503 aredescribed.

The shift register 6501 has a function of outputting signals whichselect whether to turn on or off the switches 6503. In addition, theshift register 6501 is similar to that shown in Embodiment Mode 8.

Each switch 6503 has a function of selecting whether to connect thewiring 6502 to the load 6504.

In this manner, when the shift register shown in Embodiment Mode 8 isused for the circuit shown in FIG. 62, as described above, all of thetransistors included in the circuit can be either N-channel type orP-channel type.

Note that in the circuit in FIG. 62, on/off of one switch is controlledby only one output signal of the shift register. However, on/off of aplurality of switches may be controlled by one output signal of theshift register. Thus, a configuration is described in which on/off ofthree switches is controlled by one output signal of the shift register,with reference to FIG. 63.

The circuit shown in FIG. 63 includes a shift register 6601 and aplurality of switch groups 6605. The shift register 6601 has a pluralityof output terminals OUT. Each of the switch groups 6605 has threeswitches. In addition, each of load groups 6606 has three loads.

In FIG. 63, the switch groups 6605, the load groups 6606, and the outputterminals OUT of a first stage, a second stage, a third stage, and ann-th stage are shown. In addition, n is a natural number not less thantwo.

The shift register 6601 is similar to that shown in Embodiment Mode 8.

As shown in the circuit in FIG. 63, a wiring 6602, a wiring 6603, and awiring 6601 are connected to the three loads inclnded in each load group6606 through the three switches included in each switch group 6605. Inaddition, the three switches included in each switch group 6605 arecontrolled by the shift register 6601.

A transmission signal 1 is supplied to the wiring 6602, a transmissionsignal 2 is supplied to the wiring 6603, and a transmission signal 3 issupplied to the wiring 6604. The transmission signals 1, 2, and 3 may beeither current or voltage.

Note that a plurality of control signals and various power supplypotentials are supplied to the shift register 6601, though not shown.

Next, an operation of the circuit shown in FIG. 63 is described.

The shift register 6601 sequentially outputs H-level signals or L-levelsignals from an output terminal OUT (1) of the first stage. At the sametime, the three switches included in each switch group 6605 are turnedon at the same timing, sequentially from the first stage. Then, thetransmission signals 1, 2, and 3 are sequentially supplied to the loadsincluded in each load group 6606 through the switch group 6505 from thefirst stage.

Note that when H-level signals are sequentially output from the outputterminal OUT (1) of the first stage of the shift register 6601,N-channel transistors are used as the switches included in the switchgroups 6605. On the other hand, when L-level signals are sequentiallyoutput from the output terminal OUT (1) of the first stage of the shiftregister 6601, P-channel transistors are used as the switches includedin the switch groups 6605.

In the circuit in FIG. 63, when the transmission signals 1, 2, and 3 arechanged at on/off timing of the switches included in each switch group6605, different voltages or currents can be supplied to the loadsincluded in each load group 6606.

Here, the functions of the shift register 6601 and the switch groups6605 are described.

The shift register 6601 has a function of outputting signals whichselect whether to turn on or off the switches included in the switchgroups 6605 at the same time. In addition, the shift register 6601 issimilar to that shown in Embodiment Mode 8.

Each switch group 6605 has a function of selecting whether to connectthe wiring 6602, the wiring 6603, and the wiring 6604 to the load group6606.

In this manner, in the circuit shown in FIG. 63, on/off of a pluralityof switches can be controlled by using one output signal of the shiftregister 6601. In addition, as described above, when the shift registerin Embodiment Mode 8 is used, all of the transistors included in thecircuit can be either N-channel type or P-channel type.

Here, another configuration which can employ the shift register shown inEmbodiment Mode 8, which differs from those shown in FIGS. 62 and 63 isdescribed, with reference to FIG. 64.

The circuit shown in FIG. 64 includes a shift register 6701 and aplurality of switch groups 6705. The shift register 6701 has threeoutput terminals OUT. Each of the switch groups 6705 has three switches.In addition, each of load groups 6706 has three loads.

In FIG. 64, the switch groups 6705 and the load groups 6706 of a firststage, a second stage, a third stage, and an n-th stage are shown.

The shift register 6701 is the same as that shown in Embodiment Mode 8.

As shown in the circuit in FIG. 64, a plurality of wirings 6707 are eachconnected to the three loads included in each load group 6706 throughthe three switches included in each switch group 6705. In addition, thethree switches included in each switch group 6705 are controlled by theshift register 6701.

An output signal from an output terminal OUT(1) of the first stage ofthe shift register 6701 is supplied to a wiring 6702. An output signalfrom an output terminal OUT(2) of the second stage of the shift register6701 is supplied to a wiring 6703. An output signal from an outputterminal OUT(3) of the third stage of the shift register 6701 issupplied to a wiring 6704.

In addition, a transmission signal 1 is supplied to a wiring 6707(1) ofthe first stage, a transmission signal 2 is supplied to a wiring 6707(2)of the second stage, and a transmission signal 3 is supplied to a wiring6707(3) of the third stage. The transmission signals 1, 2, and 3 may beeither current or voltage.

Note that a plurality of control signals and various power supplypotentials are supplied to the shift register 6701, though not shown.

Next, an operation of the circuit shown in FIG. 64 is described.

The shift register 6701 sequentially outputs H-level signals or L-levelsignals from an output terminal OUT (1) of the first stage. At the sametime, the switches included in each switch group 6705 are turned on oneby one, sequentially from the first stage. Therefore, one transmissionsignal is sequentially supplied to the loads included in each load group6706.

Note that when H-level signals are sequentially output from the outputterminal OUT (1) of the first stage of the shift register 6701,N-channel transistors are used as the switches included in the switchgroups 6705. On the other hand, when L-level signals are sequentiallyoutput from the output terminal OUT (1) of the first stage of the shiftregister 6701, P-channel transistors are used as the switches includedin the switch groups 6705.

In the circuit in FIG. 64, when each transmission signal is changed aton/off timing of the switches included in each switch group 6705,different voltages or currents can be supplied to the loads included ineach load group 6706.

In this manner, in the circuit shown in FIG. 64, the number oftransmission signals can be reduced by supplying one transmission signalto a plurality of loads. In FIG. 64, the number of transmission signalscan be reduced to ⅓ because three switches are provided in each switchgroup.

In addition, as described above, when the shift register in EmbodimentMode 8 is used, all of the transistors included in the circuit can beeither N-channel type or P-channel type.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 10

This embodiment mode will describe a layout diagram of the flip-flopcircuit described in Embodiment Mode 3, with reference to FIG. 65.

FIG. 65 is a layout diagram of the flip-flop circuit shown in FIG. 27.Note that the layout diagram of the flip-flop circuit shown in FIG. 65shows the case where a polycrystalline semiconductor (polysilicon) isused for a semiconductor layer of transistors. In addition, the casewill be described with reference to FIG. 65 in which a semiconductorlayer 6801, a gale electrode layer 6802, and a wiring layer 6803 areformed.

In the layout diagram of the flip-flop circuit in FIG. 65, transistors2701 to 2708 are arranged.

Note that in the layout diagram of the flip-flop circuit in FIG. 65, thetransistor 2705 has a dual-gate structure.

A wiring 2709 is disposed between each transistor and wirings 2711 a and2711 b. This is because, signals supplied to the wirings 2711 a and 2711b could be noise, which in turn could adversely affect the operation ofeach transistor. Therefore, by disposing the wiring 2709 between eachtransistor and the wirings 2711 a and 2711 b, noise can be suppressed.

Next, FIG. 66 shows a layout diagram of a flip-flop circuit in which anamorphous semiconductor (amorphous silicon) is used.

Note that the wiring 2709 is disposed between each transistor and thewirings 2711 a and 2711 b. This is because, signals supplied to thewirings 2711 a and 2711 b could be noise, which in turn could adverselyaffect the operation of each transistor. Therefore, by disposing thewiring 2709 between each transistor and the wirings 2711 a and 2711 b,noise can be suppressed.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 11

This embodiment mode will describe an example of a panel in which aplurality of pixels are formed, with reference to FIGS. 75A and 75B. InFIG. 75A, a panel 191 includes a pixel portion 591 where a plurality ofpixels 590 are arranged in matrix. The pixel portion 591 can have anactive matrix arrangement in which a switching element such as a thinfilm transistor is disposed in each pixel 590. As a display mediumprovided in the pixel 590, a light-emitting element such as anelectroluminescence element or a liquid crystal element can be used.

Note that as shown in FIG. 75B, driver circuits for driving the pixelportion 591 may be provided over the same substrate as the pixel portion591. In FIG. 75B, portions that are the same as those in FIG. 75A aredenoted by the same reference numerals as those in FIG. 75A, and theirdescription will be omitted. In FIG. 75B, a source driver 593 and a gatedriver 594 are shown as the driver circuits. Note that the invention isnot limited to this, and another driver circuit may be provided inaddition to the source driver 593 and the gate driver 594.Alternatively, the driver circuits may be formed using a differentsubstrate and mounted on the substrate where the pixel portion 591 isformed. For example, the pixel portion 591 may be formed with thin filmtransistors using a glass substrate, and the driver circuits may beformed using single crystalline substrates so that the IC chips may beconnected to the glass substrate by COG (Chip On Glass). Alternatively,the IC chips may be connected to the glass substrate by TAB (TapeAutomated Bonding) or by using a printed board.

The driver circuits may be formed over the same substrate as the pixelportion 591, using thin film transistors that are formed through thesame process as the thin film transistors included in the pixels 590. Achannel formation region of each thin film transistor may be formedusing either a polycrystalline semiconductor or an amorphoussemiconductor.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 12

FIG. 76A shows a configuration example of the pixel portion 591 shown inFIGS. 75A and 75B (hereinafter referred to as a first pixelconfiguration). The pixel portion 591 includes a plurality of sourcesignal lines S1 to Sp (p is a natural number), a plurality of scan linesG1 to Gq (q is a natural number) provided so as to intersect theplurality of source signal lines S1 to Sp, and a pixel 690 provided ateach intersection of the source signal lines S1 to Sp and the scan linesG1 to Gq.

FIG. 76B shows a configuration of the pixel 690 in FIG. 76A. In FIG.76B, the pixel 690, which is formed at the intersection of one sourceline Sx (x is a natural number not greater than p) among the pluralityof source signal lines S1 to Sp and one scan line Gy (y is a naturalnumber not greater than q) among the plurality of scan lines G1 to Gy,is shown. The pixel 690 includes a first transistor 691, a secondtransistor 692, a capacitor 693, and a light-emitting element 694. Notethat this embodiment mode shows an example where the light-emittingelement 694 has a pair of clectrodcs and emits light with a currentflowed between the pair of electrodes. In addition, parasiticcapacitance of the second transistor 692 or the like can be activelyutilized as the capacitor 693. The first transistor 691 and the secondtransistor 692 may be either N-channel transistors or P-channeltransistors. As the transistors included in the pixel 690, thin filmtransistors can be used.

A gate of the first transistor 691 is connected to the scan line Gy, oneof a source and a drain of the first transistor 691 is connected to thesource signal line Sx, and the other is connected to a gate of thesecond transistor 692 and one of electrodes of the capacitor 693. Theother electrode of the capacitor 693 is connected to a terminal 695which is supplied with a potential V3. One of a source and a drain ofthe second transistor 692 is connected to one of electrodes of thelight-emitting element 694 and the other is connected to a terminal 696which is supplied with a potential V2. The other electrode of thelight-emitting element 694 is connected to a terminal 697 which issupplied with a potential V1.

A display method of the pixel portion 591 shown in FIGS. 76A and 76B isdescribed.

One of the plurality of scan lines G1 to Gq is selected. While the scanline is selected, video signals are input to all of the plurality ofsource signal lines S1 to Sp. In this manner, video signals are inputinto one row of pixels in the pixel portion 591. By sequentiallyselecting the plurality of scan lines G1 to Gq and performing a similaroperation, video signals are input into all of the pixels 690 in thepixel portion 591.

The operation of the pixel 690, which receives a video signal from onesource signal line Sx among the plurality of source signal lines S1 toSp upon selection of one scan line Gy among the plurality of scan linesG1 to Gq, will be described. When the scan line Gy is selected, thefirst transistor 691 is turned on. An “on” state of a transistor means asource and a drain thereof are connected, while an “off” state of atransistor means a source and a drain thereof are not connected. Whenthe first transistor 691 is turned on, a video signal input to thesource signal line Sx is input to the gate of the second transistor 692through the first transistor 691. On/off states of the second transistor692 are selected based on the video signal input. When an on-state ofthe second transistor 692 is selected, the drain current of the secondtransistor 692 flows into the light-emitting element 694 so that thelight-emitting element 694 emits light.

The potential V2 and the potential V3 have a potential difference whichis kept at a constant level when the second transistor 692 is on. Thepotential V2 and the potential V3 may also have the same level. When thepotential V2 and the potential V3 are set at the same level, theterminal 695 and the terminal 696 may be connected to the same wiring.The potential V1 and the potential V2 are set to have a predeterminedpotential difference when the light-emitting element 694 is selected toemit light. In this manner, a current is flowed into the light-emittingelement 694 so that the light-emitting element 694 emits light.

Note that the wirings and electrodes are formed using one or moreelements selected from among aluminum (Al), tantalum (Ta), titanium(T1), molybdenum (Mo), tungsten (W), neodymium (Nd), chromium (Cr),nickel (Ni), platinum (Pt), gold (Au), silver (Ag), copper (Cu),magnesium (Mg), scandium (Sc), cobalt (Co), zinc (Zn), niobium (Nb),silicon (Si), phosphorus (P), boron (B), arsenic (As), gallium (Ga),indium (In), tin (Sn), and oxygen (O); a compound or alloy materialcontaining one or more of such elements (e.g., indium tin oxide (ITO),indium zinc oxide (IZO), indium tin oxide doped with silicon oxide(ITSO), zinc oxide (ZnO), aluminum neodymium (Al—Nd), or magnesiumsilver (Mg—Ag)); a substance obtained by combining such compounds; orthe like. Alternatively, a compound of the above-described material andsilicon (silicide) (e.g., aluminum silicon, molybdenum silicon, ornickel silicide) or a compound of the above-described material andnitride (e.g., titanium nitride, tantalum nitride, molybdenum nitride,or the like) can be used. Note that silicon (Si) may contain an N-typeimpurity (e.g., phosphorus) or a P-type impurity (e.g., boron) in largequantities. When silicon contains such an impurity, conductivity isimproved or silicon behaves in a manner similar to normal conductors;therefore, it can be easily utilized as wirings or electrodes. Siliconmay have any of a single crystalline state, a polycrystalline state(polysilicon), and an amorphous state (amorphous silicon). When singlecrystalline silicon or polycrystalline silicon is used, resistance canbe lowered. When amorphous silicon is used, a manufacturing process canbe simplified. Note that when aluminum or silver which has highconductivity is used, a signal delay can be reduced. Further, sincealuminum and silver can be easily etched, they can be easily patternedand thus fine processing is possible. Note also that when copper whichhas high conductivity is used, a signal delay can be reduced. It is alsopreferable to use molybdenum because it does not cause problems such asdefects of materials even when it contacts silicon or an oxidesemiconductor such as ITO or IZO; it can be easily patterned and etched;and it has high heat resistance. It is also preferable to use titaniumbecause it does not cause problems such as defects of materials evenwhen it contacts silicon or an oxide semiconductor such as ITO or IZO;it can be easily patterned and etched; and it has high heat resistance.It is also preferable to use tungsten or neodymium which has high heatresistance. In particular, it is preferable to use an alloy of neodymiumand aluminum because heat resistance is improved and aluminum can hardlyhave hillocks. It is also preferable to use silicon because it can beformed at the same time as a semiconductor layer of a transistor andalso has high heat resistance. Note also that indium tin oxide (ITO),indium zinc oxide (IZO), indium tin oxide doped with silicon oxide(ITSO), zinc oxide (ZnO), and silicon (Si) have light-transmittingproperties; therefore, they can be used for a portion to transmit light,which is preferable. For example, such materials can be used as a pixelelectrode or a common electrode.

Note that the wirings and electrodes can be formed to have either asingle-layer structure or a multi-layer structure. When a single-layerstructure is employed, a manufacturing process can be simplified andalso the manufacturing time and cost can be reduced. When a multi-layerstructure is employed, on the other hand, advantages of each materialcan be effectively utilized while disadvantages of each material can bereduced, thereby wirings and electrodes with high performance can beformed. For example, when a multi-layer structure is formed so as tocontain a low-resistance material (e.g., aluminum), resistance of awiring can be lowered. In addition, when a multi-layer structure isformed so as to contain a high heat-resistance material, such as astacked-layer structure where a low heat-resistance material which hasadvantages is sandwiched between high heat-resistance materials, heatresistance of a wiring or an electrode as a whole can be increased. Forexample, it is preferable to form a stacked-layer structure where alayer containing aluminum is sandwiched between layers containingmolybdenum or titanium. In addition, when a wiring or an electrode has aportion having a direct contact with another wiring, electrode, or thelike which is made of a different material, they may adversely affecteach other. For example, there is a case where one material is mixedinto another material, thereby the properties of the materials change,which in turn hinders the original object or causes problems duringmanufacture so that the normal manufacture cannot be conducted. In sucha case, the problems can be solved by sandwiching a layer between otherlayers or covering a layer with another layer. For example, in order tocontact indium tin oxide (ITO) and aluminum with each other, it ispreferable to sandwich titanium or molybdenum between them. In addition,in order to contact silicon and aluminum with each other, it ispreferable to sandwich titanium or molybdenum between them.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 13

FIG. 77A shows a configuration example of the pixel portion 591 shown inFIGS. 75A and 75B. FIG. 77A shows a configuration (hereinafter referredto as a second pixel configuration) which differs from the first pixelconfiguration shown in Embodiment Mode 12. The pixel portion 591includes a plurality of source signal lines S1 to Sp (p is a naturalnumber); a plurality of scan lines G1 to Gq (q is a natural number) anda plurality of scan lines R1 to Rq which are provided so as to intersectthe plurality of source signal lines S1 to Sp; and a pixel 790 providedat each intersection of the source signal lines S1 to Sp, the scan linesG1 to Gq, and the scan lines R1 to Rq.

FIG. 77B shows a configuration of the pixel 790 in FIG. 77A. In FIG.77B, the pixel 790, which is formed at the intersection of one sourceline Sx (x is a natural number not greater than p) among the pluralityof source signal lines S1 to Sp, one scan line Gy (y is a natural numbernot greater than q) among the plurality of scan lines G1 to Gq, and onescan line Ry among the plurality of scan lines R1 to Rq, is shown. Notethat in the pixel with the configuration shown in FIG. 77B, portionsthat are the same as those in FIG. 76B are denoted by the same referencenumerals as those in FIG. 76B, and their description will be omitted.FIG. 77B differs from FIG. 76B in that it has a third transistor 791.The third transistor 791 may be either an N-channel transistor or aP-channel transistor. As the transistors included in the pixel 790, thinfilm transistors can be used.

A gate of the third transistor 791 is connected to the scan line Ry, oneof a source and a drain of the third transistor 791 is connected to agate of the second transistor 692 and one of electrodes of the capacitor693, and the other is connected to a terminal 792 which is supplied witha potential V4.

A display method of the pixel portion 591 shown in FIG. 77A and FIG. 77Bis described.

A method for lighting the light-emitting element 694 is the same as thatdescribed in Embodiment Mode 12. In the pixel with the configurationshown in FIGS. 77A and 77B, the light-emitting element 694 in the pixel790 can be made not to emit light regardless of a video signal inputfrom the source signal line Sx by providing the scan line Ry and thethird transistor 791. The light-emitting time of the light-emittingelement 694 in the pixel 790 can be set by a signal input to the scanline Ry. Thus, a light-emitting period, which is shorter than the periodin which all of the scan lines G1 to Gq are sequentially selected, canbe set. In this manner, short sub-frame periods can be set whenperforming display by a time-division gray scale method, and therefore,high gray scales can be expressed.

It is only necessary that the potential V4 be set at a level which canturn off the second transistor 692 when the third transistor 791 isturned on. For example, when the third transistor 791 is turned on, thepotential V4 can be set to have the same level as the potential V3. Bysetting the potentials V3 and V4 at the same level, charges held in thecapacitor 693 can be released and a voltage between the source and thegate of the second transistor 692 can be set at zero so that the secondtransistor 692 can be turned off. Note that in order to set thepotential V3 and the potential V4 at the same level, the terminal 695and the terminal 792 may be connected to the same wiring.

Note that the position of the third transistor 791 is not limited to theone shown in FIG. 77B. For example, the third transistor 791 may bedisposed in series with the second transistor 692. In such aconfiguration, by turning off the third transistor 791 by a signal inputto the scan line Ry, a current flow into the light-emitting element 694can be blocked so that the light-emitting element 694 does not emitlight.

The third transistor 791 shown in FIG. 77B can be replaced with a diode.FIG. 77C shows a pixel configuration where the third transistor 791 isreplaced with a diode. Note that in FIG. 77C, portions that are the sameas those in FIG. 77B are denoted by the same reference numerals as thosein FIG. 77B, and their description will be omitted. One of electrodes ofa diode 781 is connected to the scan line Ry and the other electrode isconnected to the gate of the second transistor 692 and one of theelectrodes of the capacitor 693.

The diode 781 delivers a current in the direction from one electrode tothe other electrode. A P-channel transistor is used as the secondtransistor 692. By increasing the potential of one of the electrodes ofthe diode 781, the gate potential of the second transistor 692 can beincreased so that the second transistor 692 can be turned off.

Although FIG. 77C shows the configuration where the diode 781 delivers acurrent in the direction from one electrode connected to the scan lineRy to the other electrode connected to the gate of the second transistor692, and a P-channel transistor is used as the second transistor 692,the invention is not limited to this. It is also possible to employ aconfiguration where the diode 781 delivers a current in the directionfrom the electrode connected to the gate of the second transistor 692 tothe electrode connected to the scan line Ry, and an N-channel transistoris used as the second transistor 692. When the second transistor 692 isan N-channel transistor, the second transistor 692 can be turned off bydropping the potential of one of the electrodes of the diode 781 so thatthe gate potential of the second transistor 692 is dropped.

As the diode 781, a diode-connected transistor may be employed. Adiode-connected transistor means a transistor having a drain and a gateconnected together. As the diode-connected transistor, either aP-channel transistor or an N-channel transistor may be used.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 14

FIG. 78A shows a configuration example (hereinafter referred to as athird pixel configuration) of the pixel portion 591 shown in FIGS. 75Aand 75B. The pixel portion 591 includes a plurality of source signallines SI to Sp (p is a natural number), a plurality of scan lines G1 toGq (q is a natural number) provided so as to intersect the plurality ofsource signal lines S1 to Sp, and a pixel 690 provided at eachintersection of the source signal lines S1 to Sp and the scan lines G1to Gq.

FIG. 78B shows a configuration of the pixel 690 in FIG. 78A. In FIG.78B, the pixel 690, which is formed at the intersection of one sourceline Sx (x is a natural number not greater than p) among the pluralityof source signal lines S1 to Sp and one scan line Gy (y is a naturalnumber not greater than q) among the plurality of scan lines G1 to Gq,is shown. In addition, a capacitive line CO is provided corresponding toeach row. The pixel 690 includes a transistor 4691, a liquid crystalelement 4692, and a capacitor 4693. The transistor 4691 may be either anN-channel transistor or a P-channel transistor. As the transistorincluded in the pixel 690, a thin film transistor can be used.

A gate of the transistor 4691 is connected to the scan line Gy, one of asource and a drain of the transistor 4691 is connected to the sourcesignal line Sx, and the other is connected to one of electrodes of theliquid crystal element 4692 and one of electrodes of the capacitor 4693.The other electrode of the liquid crystal element 4692 is connected to aterminal 4694 which is supplied with a potential V0. The other electrodeof the capacitor 4693 is connected to the capacitive line CO. Thecapacitive line CO is supplied with the same potential as the potentialV0 which is supplied to the terminal 4694.

A display method of the pixel portion 591 shown in FIG. 78A and FIG. 78Bis described.

One of the scan lines G1 to Gq is selected. While the scan line isselected, video signals are input to all of the plurality of sourcesignal lines S1 to Sp. In this manner, video signals are input into onerow of pixels in the pixel portion 591. By sequentially selecting theplurality of scan lines G1 to Gq and performing a similar operation,video signals are input into all of the pixels 690 in the pixel portion591.

The operation of the pixel 690, which receives a video signal from onesource signal line Sx among the plurality of source signal lines S1 toSp upon selection of one scan line Gy among the plurality of scan linesG1 to Gq, will be described. When the scan line Gy is selected, thetransistor 4691 is turned on. An “on” state of a transistor means asource and a drain thereof are connected, while an “off” state of atransistor means a source and a drain thereof are not connected. Whenthe transistor 4691 is turned on, a video signal input to the sourcesignal line Sx is input to one of the electrodes of the liquid crystalelement 4692 and one of the electrodes of the capacitor 4693 through thetransistor 4691. In this manner, a voltage (which corresponds to apotential difference between the potential of the input video signal andthe potential V0 at the terminal 4694) is applied between the pair ofelectrodes of the liquid crystal element 4692, thereby the transmittanceof the liquid crystal element 4692 changes.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 15

In this embodiment mode, an example where pixels are actually formed isdescribed. FIG. 67A and FIG. 67B are cross-sectional views of a pixel ofthe panel described in Embodiment Modes 12 and 13. Here, an example isshown where a TFT is used as a switching element disposed in the pixeland a light-emitting element is used as a display medium disposed in thepixel.

In FIGS. 67A and 67B, reference numeral 1000 denotes a substrate, 1001denotes a base film, 1002 denotes a semiconductor layer, 1102 denotes asemiconductor layer, 1003 denotes a first insulating film, 1004 denotesa gate electrode, 1104 denotes an electrode, 1005 denotes a secondinsulating film, 1006 denotes an electrode, 1007 denotes a firstelectrode, 1008 denotes a third insulating film, 1009 denotes alight-emitting layer, and 1010 denotes a second electrode. Referencenumeral 1100 denotes a TFT, 1011 denotes a light-emitting element, and1101 denotes a capacitor. In FIGS. 67A and 67B, the TFT 1100 and thecapacitor 1101 are shown as typical examples of the elements included inthe pixel. The structure of FIG. 67A is described first.

As the substrate 1000, a glass substrate made of barium borosilicateglass, alumino borosilicate glass, or the like; a quartz substrate; aceramic substrate; or the like can be used. Alternatively, a metalsubstrate including stainless steel or a semiconductor substrate eachhaving an insulating film formed on its surface can be used. A substratemade of a flexible synthetic resin such as plastic can also be used. Thesurface of the substrate 1000 may be planarized by polishing, e.g., aCMP method.

As the base film 1001, an insulating film made of silicon oxide, siliconnitride, silicon nitride oxide, or the like can be used. By providingthe base film 1001, an alkaline metal such as Na or an alkaline earthmetal contained in the substrate 1000 can be prevented from diffusinginto the semiconductor layer 1002, which would otherwise adverselyaffect the characteristics of the TFT 1100. Although the base film 1001in FIGS. 67A and 67B has a single-layer structure, a plurality of layersof two or more layers can be used. Note that when there is littleconcern about the diffusion of impurities in the case of using a quartzsubstrate, for example, the base film 1001 is not necessarily provided.

As the semiconductor layer 1002 and the semiconductor layer 1102, acrystalline semiconductor film or an amorphous semiconductor film whichhas been processed into a predetermined shape can be used. A crystallinesemiconductor film can be obtained by crystallizing an amorphoussemiconductor film. As a crystallization method, a laser crystallizationmethod, a thermal crystallization method using RTA or an annealingfurnace, a thermal crystallization method using a metal element whichpromotes crystallization, or the like can be used. The semiconductorlayer 1002 includes a channel formation region and a pair of impurityregions doped with an impurity element which imparts a conductivitytype. Note that impurity regions which are doped with an impurityelement at a low concentration (LDD regions) may also be providedbetween the channel formation region and the pair of impurity regions.The semiconductor layer 1102 can have a structure in which the wholeregion is doped with impurity elements which impart conductivity types.

As the first insulating film 1003, silicon oxide, silicon nitride,silicon nitride oxide, or the like can be used, and either a singlelayer or stacked layers of a plurality of films can be used.

Note that a film containing hydrogen may also be used as the firstinsulating film 1003 so that the semiconductor layer 1002 can behydrogenated.

For the gate electrode 1004 and the electrode 1104, an element selectedfrom among Ta, W, Ti, Mo, Al, Cu, Cr, and Nd, or an alloy or compoundcontaining a plurality of such elements can be used. Further, the gateelectrode 1004 and the electrode 1104 can be formed to have either asingle-layer structure or a stacked-layer structure of theabove-described materials.

The TFT 1100 includes the semiconductor layer 1002, the gate electrode1004, and the first insulating film 1003 between the semiconductor layer1002 and the gate electrode 1004. Although FIGS. 67A and 67B show onlythe TFT 1100 connected to the first electrode 1007 of the light-emittingelement 1011 as the TIT which forms the pixel, a structure having aplurality of TFT may also be employed. In addition, although the TFT1100 is illustrated as a top-gate transistor in this embodiment mode, itis also possible to employ a bottom-gate transistor having a gateelectrode below a semiconductor layer, or a dual-gate transistor havinggate electrodes above and below a semiconductor layer.

The capacitor 1101 is formed from the first insulating film 1003 as adielectric, and the semiconductor layer 1102 and the electrode 1104which are opposite each other with the first insulating film 1003interposed therebetween, as a pair of electrodes. Note that althoughFIGS. 67A and 67B show examples where the capacitor included in thepixel has the semiconductor layer 1102, which is formed at the same timeas the semiconductor layer 1002 of the TFT 1100, as one of the pair ofelectrodes and also has the electrode 1104, which is formed at the sametime as the gate electrode 1004 of the TFT 1100, as the other electrode,the invention is not limited to this structure.

As the second insulating film 1005, either a single layer or stackedlayers of an inorganic insulating film or an organic insulating film canbe used. As an inorganic insulating film, a silicon oxide film formed bya CVD method, a silicon oxide film formed by a SOG (Spin On Glass)method, or the like can be used. As an organic insulating film, a filmmade of polyimide, polyamide, BCB (benzocyclobutene), acrylic, apositive photosensitive organic resin, a negative photosensitive organicresin, or the like can be used.

In addition, for the second insulating film 1005, a material having askeletal structure with the bond of silicon (Si) and oxygen (O) can beused. As a substituent of this material, an organic group containing atleast hydrogen (e.g., an alkyl group or an aryl group) is used.Alternatively, a fluoro group may be used as the substituent. As afurther alternative, both a fluoro group and an organic group containingat least hydrogen may be used as the substituent.

Note that the surface of the second insulating film 1005 may be nitridedby high-density plasma treatment. High-density plasma is generated byusing high-frequency microwaves, e.g., 2.45 GHz. Note that as thehigh-density plasma, plasma which has an electron density of not lessthan 10¹¹ cm³ and an electron temperature of 0.2 to 2.0 eV, inclusive(preferably, 0.5 to 1.5 eV, inclusive) is used. When such high-densityplasma with a low electron temperature is used, kinetic energy ofactivated species can be low. Therefore, it is possible to form a filmwhich suffers little plasma damage and has less defects than a filmformed by the conventional plasma treatment. In the high-density plasmatreatment, the substrate 1000 is set at temperatures in the range of 350to 450° C. In addition, in an apparatus for generating high-densityplasma, the distance between an antenna which generates microwaves andthe substrate 1000 is set at 20 to 80 mm, inclusive (preferably, 20 to60 mm, inclusive).

The surface of the second insulating film 1005 is nitrided by the abovehigh-density plasma treatment under an atmosphere containing nitrogen(N₂) and a rare gas (which includes at least one of He, Ne, Ar, Kr, andXe); an atmosphere containing nitrogen, hydrogen (H₂), and a rare gas;or an atmosphere containing NH₃ and a rare gas. In the surface of thesecond insulating film 1005 formed by high-density plasma nitridationtreatment, an element such as H, He, Ne, Ar, Kr, or Xe is mixed. Forexample, a silicon oxide film or a silicon oxynitride film is used asthe second insulating film 1005, and the surface of the film is treatedwith high-density plasma so that a silicon nitride film is formed. Thesemiconductor layer 1002 of the TFT 1100 may be hydrogenated by usingthe hydrogen contained in the thusly formed silicon nitride film. Notethat the hydrogenation treatment may be combined with theabove-described hydrogenation treatment which uses hydrogen contained inthe first insulating film 1003.

Note that the second insulating film 1005 may be formed by depositinganother insulating film over the nitride film which is formed by theabove high-density plasma treatment.

The electrode 1006 can be formed using an element selected from amongAl, W, Mo, Ti, Pt, Cu, Ta, Au, and Mn, or an alloy containing aplurality of elements selected from among Al, Ni, C, W, Mo, Ti, Pt, Cu,Ta, Au, and Mn. Further, the electrode 1006 can be formed to have eithera single-layer structure or a stacked-layer structure of theabove-described materials.

One or both of the first electrode 1007 and the second electrode 1010can be formed as a transparent electrode. For the transparent electrode,indium oxide containing tungsten oxide (IWO), indium oxide containingtungsten oxide and zinc oxide (IWZO), indium oxide containing titaniumoxide (ITiO), indium tin oxide containing titanium oxide (ITTiO), or thelike can be used. Needless to say, indium tin oxide (ITO), indium zincoxide (IZO), indium tin oxide doped with silicon oxide (ITSO), or thelike can also be used.

A light-emitting element can be categorized as a light-emitting elementwhich emits light with a DC voltage applied thereto (hereinafterreferred to as a DC-drive light-emitting element) or a light-emittingelement which emits light with an AC voltage applied thereto(hereinafter referred to as an AC-drive light-emitting element).

A DC-drive light-emitting element is preferably formed to have aplurality of layers having different functions such as a holeinjection/transport layer, a light-emitting layer, and an electroninjection/transport layer.

The hole injection/transport layer is preferably formed with a compositematerial of an organic compound material having a hole transportproperty and an inorganic compound material which exhibits an electronaccepting property with respect to the organic compound material. Byemploying such a structure, many hole carriers are generated in theorganic compound which inherently has few carriers, thereby quite anexcellent hole injection/transport property can be obtained. By such aneffect, driving voltage can be lowered than that in the conventionaltechnique. Further, since the hole injecting/transport layer can beformed to be thick without causing an increase in driving voltage, shortcircuit of the light-emitting element due to dust or the like can besuppressed.

As an organic compound material having a hole transport property, thereare, for example,4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA); 1,3,5-tris[N,N-di(m-tolyl)amino]benzene(abbreviation: m-MTDAB);N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine(abbreviation: TPD); 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB); and the like. However, the invention is not limitedto these.

As an inorganic compound material which exhibits an electron acceptingproperty, there are titanium oxide, zirconium oxide, vanadium oxide,molybdenum oxide, tungsten oxide, rhenium oxide, ruthenium oxide, zincoxide, and the like. In particular, vanadium oxide, molybdenum oxide,tungsten oxide, and rhenium oxide are preferable because they can bedeposited in vacuum, and are easy to be handled.

The electron injection/transport layer is formed with an organiccompound material having an electron transport property. Specifically,there are tris(8-quinolinolato)aluminum (abbreviation: Alq₃);tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃); and thelike. However, the invention is not limited to these.

In the DC-drive light-emitting element, a light-emitting layer can beformed using, for example, 9,10-di(2-naphthyl)anthracene (abbreviation:DNA); 9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation:t-BuDNA); 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi);coumarin 30; coumarin 6; coumarin 545; coumarin 545T, perylene; rubrene;periflanthene; 2,5,8,11-tetra(tert-butyl)perylene (abbreviation: TBP);9,10-diphenylanthracene (abbreviation: DPA); 5,12-diphenyltetracene;4-(dicyanomethylene)-2-methyl-[p-(dimethylamino)styryl]-4H-pyran(abbreviation: DCM1);4-(dicyanomethylene)-2-methyl-6-[2-(julolidin-9-yl)ethenyl]-4H-pyran(abbreviation: DCM2);4-(dicyanomethylene)-2,6-bis[p-(dimethylamino)styryl]-4H-pyran(abbreviation: BisDCM); and the like. Alternatively, the followingcompounds capable of generating phosphorescence can be used:bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(picolinate)(abbreviation: FPrpic); bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(picolinate) (abbreviation:Ir(CF₃ppy)₂(pic)); tris(2-phenylpyridinato-N,C^(2′))iridium(abbreviation: Ir(ppy)₃);bis(2-phenylpyridinato-N,C^(2′))iridium(acetylacetonate) (abbreviation:Ir(ppy)₂(acac));bis[2-(2′-thieoyl)pyridinato-N,C^(3′)]iridium(acetylacetonate)(abbreviation: Ir(thp)₂(acac));bis(2-phenylquinolinato-N,C^(2′))iridium(acetylacetonate) (abbreviation:Ir(pq)²(acac));bis[2-(2′-benzothienyl)pyridinatoN,C^(3′)]iridium(acetylacetonate)(abbreviation: Ir(btp)₂(acac)); and the like.

Alternatively, as a high molecular electroluminescent material which canbe used for forming the light-emitting layer, polyparaphenylenevinylene, polyparaphenylene, polythiopbene, or polyfluorene can be used.

The other of the first electrode 1007 and the second electrode 1010 maybe formed with a material which does not transmit light. For example,alkaline metals such as Li and Cs, alkaline earth metals such as Mg, Ca,and Sr, alloys containing these (Mg:Ag, Al:Li, and Mg:In), compounds ofthese (CaF₂ and calcium nitride), or rare earth metals such as Yb and Ercan be used.

The third insulating film 1008 can be formed using a similar material tothe second insulating film 1005. The third insulating film 1008 isformed around the first electrode 1007 so as to cover the ends of thefirst electrode 1007, and has a function of separating thelight-emitting layers 1009 of adjacent pixels.

The light-emitting layer 1009 has a single layer or a plurality oflayers. When the light-emitting layer 1009 has a plurality of layers,these layers can be divided into a hole injection layer, a holetransport layer, a light-emitting layer, an electron transport layer, anelectron injection layer, and the like in terms of the carrier transportproperties. Note that the boundary of each layer is not necessarilyclear, and there may be cases where the boundary cannot be distinguishedclearly because the material which forms each layer is partially mixedinto the adjacent layeL Each layer may be formed with an organicmaterial or an inorganic material. As for an organic material, either ahigh molecular material or a low molecular material can be used.

The light-emitting element 1011 includes the light-emitting layer 1009and the first electrode 1007 and the second electrode 1010 which overlapwith each other with the light-emitting layer 1009 interposedtherebetween. One of the first electrode 1007 and the second electrode1010 corresponds to an anode and the other corresponds to a cathode.When a forward voltage which is higher than the threshold voltage of thelight-emitting element 1011 is applied between the anode and the cathodeof the light-emitting element 1011, a current flows form the anode tothe cathode so that the light-emitting element 1011 emits light.

On the other hand, the AC-drive light-emitting element has adouble-insulator structure in which a light-emitting layer which isinterposed between two insulating films is further interposed between apair of electrodes. Light emission can be obtained by applying an ACvoltage between the pair of electrodes. As a material of thelight-emitting layer of the AC-drive light-emitting element, ZnS, SrS,BaAl₂S₄, or the like can be used. As a material of the insulating filmswhich interpose the light-emitting layer therebetween, Ta₂O₅, SiO₂,Y₂O₃, BaTiO₃, SrTiO₃, silicon nitride, or the like can be used.

The structure of FIG. 67B is described. Note that portions that are thesame as those in FIG. 67A are denoted by the same reference numerals asthose in FIG. 67A, and their description will be omitted.

FIG. 67B shows a structure where an insulating film 1108 is providedbetween the second insulating film 1005 and the third insulating film1008. The electrode 1006 and the first electrode 1007 are connected toeach other with an electrode 1106 in a contact hole provided in theinsulating film 1108.

Note that the electrode 1106 is not necessarily provided. That is, thefirst electrode 1007 may be directly connected to the electrode 1006without the use of the electrode 1106. In that case, the step of formingthe electrode 1106 can be omitted so that the cost can be reduced.

When the first electrode 1007 is directly connected to the electrode1006 without the use of the electrode 1106, the coverage of theelectrode 1006 with the first electrode 1007 could be poor depending onthe material or method for forming the first electrode 1007, and theelectrode 1006 could break. In view of such circumstance, it isadvantageous, as shown in FIG. 67B, to connect the electrode 1006 andthe first electrode 1007 to each other with the electrode 1106 in thecontact hole that is provided in the insulating film 1108.

The insulating film 1108 can have a similar structure to the secondinsulating film 1005. The electrode 1106 can have a similar structure tothe electrode 1006.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 16

In this embodiment mode, an example where pixels are actually formed isdescribed. FIG. 68 is a cross-sectional view of a pixel of the panelwhich is described in Embodiment Modes 11 to 14. Here, an example isshown where a TFT is used as a switching element disposed in the pixeland a light-emitting element is used as a display medium disposed in thepixel. Note that portions that are the same as those in FIGS. 67A and67B shown in Embodiment Mode 15 are denoted by the same referencenumerals as those in FIGS. 67A and 67B, and their description will beomitted.

The pixel shown in FIG. 68 differs from FIG. 67A shown in EmbodimentMode 15 in the structures of the TFT 1100 and the capacitor 1101. FIG.68 shows an example where a bottom-gate TFT is used as the TFT 1100. TheTFT 1100 includes a gate electrode 2803; a semiconductor layer whichincludes a channel formation region 2806, LDD regions 2807, and impurityregions 2808; and a first insulating film 2805 between the gateelectrode 2803 and the semiconductor layer. The first insulating film2805 functions as a gate insulating film of the TFT 1100. The impurityregions 2808 function as a source region and a drain region of the TFT1100.

The capacitor 1101 is formed from the first insulating film 2805 as adielectric, and a semiconductor layer and an electrode 2804 which areopposite each other with the first insulating film 2805 interposedtherebetween, as a pair of electrodes. The semiconductor layer includesa channel formation region 2809, LDD regions 2810, and impurity regions2811. Note that FIG. 68 shows an example where the capacitor included inthe pixel has the semiconductor layer, which is formed at the same timeas the semiconductor layer functioning as an active layer of the TFT1100, as one of the pair of electrodes and also has the electrode 2804,which is formed at the same time as the gate electrode 2803 of the TFT1100, as the other electrode; however, the invention is not limited tothis structure.

For the semiconductor layer including the channel formation region 2806,the LDD regions 2807, and the impurity regions 2808, and thesemiconductor layer including the channel formation region 2809, the LDDregions 2810, and the impurity regions 2811, materials similar to thoseof the semiconductor layer 1002 and the semiconductor layer 1102 inFIGS. 67A and 67B can be used. For the gate electrode 2803 and theelectrode 2804, a material similar to that of the gate electrode 1004 inFIGS. 67A and 67B can be used.

The channel formation region 2806 and the channel formation region 2809may be doped with an impurity element which imparts a conductivity type.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 17

In this embodiment mode, an example where pixels are actually formed isdescribed. FIGS. 69A and 69B are cross-sectional views of a pixel of thepanel which is described in Embodiment Modes 13 and 14. Here, an exampleis shown where a TFT is used as a switching element disposed in thepixel and a light-emitting element is used as a display medium disposedin the pixel. Note that portions that are the same as those in FIGS. 67Aand 67B shown in Embodiment Mode 15 are denoted by the same referencenumerals as those in FIGS. 67A and 67B, and their description will beomitted.

The pixels shown in FIGS. 69A and 69B differ from FIG. 67A shown inEmbodiment Mode 15 in the structures of the TFT 1100 and the capacitor1101. FIG. 69A shows an example where a bottom-gate TFT with achannel-etched structure is used as the TFT 1100. FIG. 69B shows anexample where a bottom-gate TFT with a channel-protective structure isused as the TFT 1100. The TFT 1100 with the channel-protective structureshown in FIG. 69B differs from the TFT 1100 with the channel-etchedstructure shown in FIG. 69A in that an insulator 3001 serving as anetching mask is provided over a region of the semiconductor layer 2906in which a channel is formed.

In FIGS. 69A and 69B, the TFT 1100 includes a gate electrode 2993, afirst insulating film 2905 over the gate electrode 2993, a semiconductorlayer 2906 over the first insulating film 2905, and N-type semiconductorlayers 2908 and 2909 over the semiconductor layer 2906. The firstinsulating film 2905 functions as a gate insulating film of the TFT1100. The N-type semiconductor layers 2908 and 2909 function as a sourceand a drain of the TFT 1100. Electrodes 2911 and 2912 are formed overthe N-type semiconductor layers 2908 and 2909, respectively. One end ofthe electrode 2911 extends to a region where the semiconductor layer2906 is not formed, and in that region, the electrode 1006 is formed incontact with the top portion of the electrode 2911.

The capacitor 1101 is formed from the first insulating film 2905 as adielectric; an electrode 2904 as one of the electrodes; and asemiconductor layer 2907 which is opposite the electrode 2904 with thefirst insulating film 2905 interposed therebetween, an N-typesemiconductor layer 2910 over the semiconductor layer 2907, and anelectrode 2913 over the N-type semiconductor layer 2910 as the otherelectrode. The electrode 2904 can be formed at the same time as the gateelectrode 2993. The semiconductor layer 2907 can be formed at the sametime as the semiconductor layer 2906. The N-type semiconductor layer2910 can be formed at the same time as the N-type semiconductor layers2908 and 2909. The electrode 2913 can be formed at the same time as theelectrodes 2911 and 2912.

For the gate electrode 2993 and the electrode 2904, a material similarto that of the gate electrode 1004 in FIGS. 67A and 67B can be used. Forthe semiconductor layers 2906 and 2907, amorphous semiconductor filmscan be used. For the first insulating film 2905, a material similar tothat of the first insulating film 1003 in FIGS. 67A and 67B can be used.For the electrodes 2911, 2912, and 2913, a material similar to that ofthe electrode 1006 can be used. For the N-type semiconductor layers2910, 2908, and 2909, semiconductor films containing N-type impurityelements can be used.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 18

In this embodiment mode, an example where pixels are actually formed isdescribed. FIGS. 70A to 70C are cross-sectional views of a pixel of thepanel which is described in Embodiment Mode 14. Here, an example isshown where a TFT is used as a switching element disposed in the pixeland a liquid crystal element is used as a display medium disposed in thepixel.

The pixels shown in FIGS. 70A, 70B, and 70C each show a structure wherea liquid crystal element is provided instead of the light-emittingelement 1011 in the 16 structures shown in FIGS. 67A and 67B ofEmbodiment Mode 15 and the structure shown in FIG. 68 of Embodiment Mode16. Portions that are the same as those in FIGS. 67A, 67B, and 68 aredenoted by the same reference numerals as those in FIGS. 67A, 67B, and68, and their description will be omitted.

The liquid crystal element includes a first electrode 4000, an alignmentfilm 4001 formed over the first electrode 4000, a liquid crystal layer4002, an alignment film 4003, and a second electrode 4004. When avoltage is applied between the first electrode 4000 and the secondelectrode 4004, orientation of liquid crystals changes, thereby thetransmittance of the liquid crystal element changes. The secondelectrode 4004 and the alignment film 4003 are formed on a countersubstrate 4005.

One or both of the first electrode 4000 and the second electrode 4004can be formed as a transparent electrode. For the transparent electrode,indium oxide containing tungsten oxide (IWO), indium oxide containingtungsten oxide and zinc oxide (IWZO), indium oxide containing titaniumoxide (ITiO), indium tin oxide containing titanium oxide (ITTiO), or thelike can be used. Needless to say, indium tin oxide (ITO), indium zincoxide (IZO), indium tin oxide doped with silicon oxide (ITSO), or thelike can also be used. The other of the first electrode 4000 and thesecond electrode 4004 may be formed with a material which does nottransmit light. For example, alkaline metals such as LI and Cs, alkalineearth metals such as Mg, Ca, and Sr, alloys containing these (Mg:Ag,Al:Li, and Mg:In), compounds of these (CaF₂ and calcium nitride), rareearth metals such as Yb and Er can be used.

For the liquid crystal layer 4002, known liquid crystals can be freelyused. For example, ferroelectric liquid crystals or antiferroelectricliquid crystals can be used for the liquid crystal layer 4002. Inaddition, as a driving method of the liquid crystals, a TN (TwistedNematic) mode, an MVA (Multi-domain Vertical Alignment) mode, an ASM(Axially Symmetric aligned Micro-cell) mode, an OCB (Optical CompensatedBend) mode, or the like can be freely used.

Although this embodiment mode has illustrated the example where a pairof electrodes (the first electrode 4000 and the second electrode 4004)which apply a voltage to the liquid crystal layer 4002 are formed ondifferent substrates, the invention is not limited to this. The secondelectrode 4004 may be formed on the substrate 1000. Then, an IPS(In-Plane-Switching) mode may be used as the driving method of theliquid crystals. In addition, one or both of the alignment film 4001 andthe alignment film 4003 may be omitted depending on the material of theliquid crystal layer 4002.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 19

In this embodiment mode, an example where pixels are actually formed isdescribed. FIGS. 71A and 71B are cross-sectional views of a pixel of thepanel which is described in Embodiment Mode 14. Here, an example isshown where a TFT is used as a switching element disposed in the pixeland a liquid crystal element is used as a display medium disposed in thepixel.

The pixels shown in FIGS. 71A and 71B each show a structure where aliquid crystal element is provided instead of the light-emitting element1011 in the structures shown in FIGS. 69A and 69B of Embodiment Mode 17.Portions that are the same as those in FIGS. 69A and 69B are denoted bythe same reference numerals as those in FIGS. 69A and 69B, and theirdescription will be omitted. In addition, the structure of the liquidcrystal element and the like are similar to the structures shown inFIGS. 70A to 70C of Embodiment Mode 17; therefore, their descriptionwill be omitted.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 20

This embodiment mode will describe a structure where a substrate overwhich pixels are formed is sealed. FIG. 72A is a top view of a panelformed by sealing a substrate over which pixels are formed, and FIGS.72B and 72C are cross-sectional views along a line A-A′ of FIG. 72A.FIGS. 72B and 72C show examples where sealing is performed by usingdifferent methods.

In FIGS. 72A to 72C, a pixel portion 1402 having a plurality of pixelsis disposed over a substrate 1401, a sealant 1406 is provided so as tosurround the pixel portion 1402, and a sealant 1407 is attached to thesubstrate 1401. For the structure of the pixels, the structure shown inEmbodiment Mode 16, 17, or 18 can be used.

In the display panel in FIG. 72B, the sealant 1407 corresponds to acounter substrate 1421. The counter substrate 1421 which is transparentis attached to the substrate 1401, using the sealant 1406 as an adhesivelayer. A hermetically sealed space 1422 is formed by the substrate 1401,the counter substrate 1421, and the sealant 1406. The counter substrate1421 is provided with color filters 1420 and a protective film 1423 forprotecting the color filters. Light emitted from light-emitting elementsprovided in the pixel portion 1402 is emitted outside through the colorfilters 1420. The hermetically sealed space 1422 is filled with an inertresin, liquid, or the like. Note that as a resin for filling thehermetically sealed space 1422, a light-transmissive resin in which anabsorbent is dispersed may be used. Alternatively, the same material maybe used for the sealant 1406 and the material for filling thehermetically sealed space 1422, so that the attachment of the countersubstrate 1421 and the sealing of the pixel portion 1402 can beconducted at the same time.

In the display panel shown in FIG. 72C, the sealant 1407 corresponds toa sealant 1424. The sealant 1424 is attached to the substrate 1401 usingthe sealant 1406 as an adhesive layer A hermetically scaled space 1408is formed by the substrate 1401, the sealant 1406, and the sealant 1424.The sealant 1424 is provided with an absorbent 1409 in its recessedportion in advance, and inside the hermetically sealed space 1408, theabsorbent 1409 functions to keep a clean atmosphere by adsorbingmoisture, oxygen, or the like and suppress deterioration oflight-emitting elements. The recessed portion is covered with a finelymeshed covering material 1410, and the covering material 1410 transmitsair and moisture but does not transmit the absorbent 1409. Thehermetically sealed space 1408 may be filled with a rare gas such asnitrogen or argon or with an inert resin or liquid.

On the substrate 1401, an input terminal portion 1411 for transmittingsignals to the pixel portion 1402 and the like are provided. Signalssuch as video signals are transmitted to the input terminal portion 1411through an FPC (Flexible Printed Circuit) 1412. At the input terminalportion 1411, wirings formed on the substrate 1401 and wirings providedin the FPC (Flexible Printed Circuit) 1412 are electrically connected toeach other with a resin in which conductors are dispersed (ananisotropic conductive rein: ACF).

Driver circuits for inputting signals to the pixel portion 1402 may beformed over the same substrate 1401 as the pixel portion 1402.Alternatively, the driver circuits for inputting signals to the pixelportion 1402 may be formed on IC chips, and the IC chips may beconnected to the substrate 1401 by COG (Chip On Glass), or the IC chipsmay be disposed on the substrate 1401 by TAB (Tape Automated Bonding) orby using a printed board.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 21

The invention can be applied to a display module in which circuits forinputting signals to a panel are mounted on the panel.

FIG. 73 shows a display module combining a panel 980 and a circuit board984. Although FIG. 73 shows an example where a controller circuit 985, asignal divider circuit 986, and the like are formed over the circuitboard 984, the circuits formed over the circuit board 984 are notlimited to these. Any circuits which can generate signals forcontrolling the panel may be formed.

Signals output from the circuits formed over the circuit board 984 areinput to the panel 980 through a connection wiring 987.

The panel 980 includes a pixel portion 981, a source driver 982, and agate driver 983. The panel 980 can have a configuration that is similarto any of those shown in Embodiment Modes 11 to 14. Although FIG. 73shows an example where the source driver 982 and the gate driver 983 areformed over the same substrate as the pixel portion 981, the displaymodule of the invention is not limited to this. Only the gate driver 983may be formed over the same substrate as the pixel portion 981, whilethe source driver 982 may be formed over the circuit board.Alternatively, both of the source driver 982 and the gate driver 983 maybe formed over the circuit board.

Display portions of various electronic devices can be formed by usingsuch a display module.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 22

The invention can be applied to various electronic devices. Examples ofelectronic devices include cameras (e.g., video cameras or digitalcameras), projectors, head mounted displays (e.g., goggle displays),navigation systems, car stereos, personal computers, game machines,portable information terminals (e.g., mobile computers, mobile phones,or electronic books), image reproducing devices provided with recordingmedia, and the like. As an example of image reproducing devices providedwith recording media, there is a device which reproduces the content ofa recording medium such as a digital versatile disc (DVD) and has adisplay for displaying the reproduced image, or the like. FIGS. 74A to74D exemplarily illustrate such electronic devices.

FIG. 74A shows a laptop personal computer, which includes a main body911, a housing 912, a display portion 913, a keyboard 914, an externalconnection port 915, a pointing device 916, and the like. The inventionis applied to the display portion 913. By using the invention, powerconsumption of the display portion can be reduced.

FIG. 74B shows an image reproducing device provided with a recordingmedium (specifically, a DVD player), which includes a main body 921, ahousing 922, a first display portion 923, a second display portion 924,a recording medium (e.g., DVD) reading portion 925, operating keys 926,speaker portions 927, and the like. The first display portion 923 mainlydisplays image data, while the second display portion 924 mainlydisplays text data. The invention is applied to the first displayportion 923 and the second display portion 924. By using the invention,power consumption of the display portion can be reduced.

FIG. 74C shows a mobile phone, which includes a main body 931, an audiooutput portion 932, an audio input portion 933, a display portion 934,operating switches 935, an antenna 936, and the like. The invention isapplied to the display portion 934. By using the invention, powerconsumption of the display portion can be reduced.

FIG. 74D shows a camera, which includes a main body 941, a displayportion 942, a housing 943, an external connection port 944, a remotecontroller receiving portion 945, an image receiving portion 946, abattery 947, an audio input portion 948, operating keys 949, and thelike. The invention is applied to the display portion 942. By using theinvention, power consumption of the display portion can be reduced.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

Embodiment Mode 23

This embodiment mode will describe examples where a display device withthe pixel configuration of the invention is applied to a display portionof a display panel, with reference to the drawings. A display panelwhose display portion has a display device with the pixel configurationof the invention can be incorporated in a moving object, a building, orthe like.

FIGS. 41A and 41B each show a moving object incorporating a displaydevice, as an exemplary display panel whose display portion has adisplay device with the pixel configuration of the invention. FIG. 41Ashows a display panel 9702 which is attached to a glass door in a traincar body 9701, as an exemplary moving object incorporating a displaydevice. The display panel 9702 shown in FIG. 41A whose display portionhas a display device with the pixel configuration of the invention caneasily switch images displayed on the display portion in response toexternal signals. Therefore, images on the display panel can beperiodically switched in accordance with the time cycle through whichpassengers' ages or sex vary, thereby more efficient advertising effectscan be expected.

Note that the position for setting the display panel whose displayportion has a display device with the pixel configuration of theinvention is not limited to a glass door of a train car body as shown inFIG. 41A, and thus the display panel can be provided anywhere bychanging the shape of the panel. FIG. 41B shows an example thereof.

FIG. 41B shows an interior view of a train car body. In FIG. 41B,display panels 9703 attached to glass windows and a display panel 9704hung on the ceiling are shown in addition to the display panels 9702attached to the glass doors shown in FIG. 41A. The display panels 9703having the pixel configuration of the invention have self-luminousdisplay elements. Therefore, by displaying advertisement images in rushhours, while displaying no images in off-peak hours, outside views canbe seen by passengers through the train windows. In addition, thedisplay panel 9704 having the pixel configuration of the invention canbe flexibly bent by providing self-luminous display elements andswitching elements such as organic transistors over a film-formsubstrate, and images can be displayed on the display panel 9704 bydriving the self-luminous display elements.

Another example where a display panel whose display portion has adisplay device with the pixel configuration of the invention is appliedto a moving object incorporating a display device is described, withreference to FIG. 42.

FIG. 42 shows a moving object incorporating a display device, as anexemplary display panel whose display portion has a display device withthe pixel configuration of the invention. FIG. 42 shows a display panel9901 which is incorporated in a body 9902 of a car, as an exemplarymoving object incorporating a display device. The display panel 9901shown in FIG. 42 whose display portion has a display device with thepixel configuration of the invention is incorporated in the body of thecar, and displays information on the operation of the car or informationinput from outside of the car on an on-demand basis. Further, it has anavigation function to a destination of the car.

Note that the position for setting the display panel whose displayportion has a display device with the pixel configuration of theinvention is not limited to a front portion of a car body as shown inFIG. 42, and thus the display panel can be provided anywhere such asglass windows or doors by changing the shape of the panel.

Another example where a display panel whose display portion has adisplay device with the pixel configuration of the invention is appliedto a moving object incorporating a display device is described, withreference to FIGS. 43A and 43B.

FIGS. 43A and 43B each show a moving object incorporating a displaydevice, as an exemplary display panel whose display portion has adisplay device with the pixel configuration of the invention. FIG. 43Ashows a display panel 10102 which is incorporated in a part of theceiling above the passenger's seat inside an airplane body 10101, as anexemplary moving object incorporating a display device. The displaypanel 10102 shown in FIG. 43A whose display portion has a display devicewith the pixel configuration of the invention is fixed to the airplanebody 10101 with a hinge portion 10103, so that passengers can see thedisplay panel 10102 with the help of a telescopic motion of the hingeportion 10103. The display panel 10102 has a function of displayinginformation as well as a function of an advertisement or amusement meanswith the operation of passengers. In addition, by storing the displaypanel 10102 in the airplane body 10101 by folding the hinge portion10103 back on the ceiling as shown in FIG. 43B, safety during theairplane's takeoff and landing can be secured. Note that by lightingdisplay elements of the display panel in an emergency, the display panelcan be also utilized as a guide light.

Note that the position for setting the display panel whose displayportion has a display device with the pixel configuration of theinvention is not limited to the ceiling of the airplane body 10101 shownin FIGS. 43A and 43B, and thus the display panel can be providedanywhere such as seats or doors by changing the shape of the panel. Forexample, the display panel may be set on the backside of a seat so thata passenger on the rear seat can operate and view the display panel.

Although this embodiment mode has illustrated a train car body, a carbody, and an airplane body as exemplary moving objects, the invention isnot limited to these, and the invention can be applied to motorbikes,four-wheeled vehicles (including cars, buses, and the like), trains(including monorails, railroads, and the like), ships and vessels, andthe like. By employing a display panel whose display portion has thepixel configuration of the invention, reduction in size and powerconsumption of the display panel can be achieved, and a moving objecthaving a display medium which can operate excellently can be provided.In particular, since images that are displayed on a plurality of displaypanels incorporated in a moving object can be switched all at once, theinvention is quite advantageous in that it can be applied to advertisingmedia for unspecified number of customers, or information display boardsin an emergency.

An example where a display panel whose display portion has a displaydevice with the pixel configuration of the invention is applied to astructure is described, with reference to FIG. 53.

FIG. 53 illustrates an example where a flexible display panel is formedby providing self-luminous display elements and switching elements suchas organic transistors over a film-form substrate, and images can bedisplayed on the display panel by driving the self-luminous displayelements, as an exemplary display panel whose display portion has adisplay device with the pixel configuration of the invention. In FIG.53, a display panel is provided on a curved surface of an outsidecolumnar object such as a telephone pole as a structure, andspecifically, shown here is a structure where display panels 9802 areattached to telephone poles 9801 which are columnar objects.

The display panels 9802 shown in FIG. 53 are positioned at about a halfheight of the telephone poles, so as to be higher than the eye level ofhumans. When the display panels are viewed from a moving object 9803,images on the display panels 9802 can be recognized. By displaying thesame images on the display panels 9802 that are provided on the outsidetelephone poles which stand together in large numbers, viewers canrecognize the displayed information or advertisement. The display panels9802 provided on the telephone poles 9801 in FIG. 53 can easily displaythe same images by using external signals; therefore, quite efficientinformation display and advertising effects can be expected. Inaddition, when self-luminous display elements are provided as thedisplay elements in the display panel of the invention, the displaypanel can be effectively used as a highly visible display medium even atnight.

Another example where a display panel whose display portion has adisplay device with the pixel configuration of the invention is appliedto a structure is described with reference to FIG. 54, which differsfrom FIG. 53.

FIG. 54 shows another application example of a display panel whosedisplay portion has a display device with the pixel configuration of theinvention. In FIG. 54, an example of a display panel 10001 which isincorporated in the sidewall of a prefabricated bath unit 10002 isshown. The display panel 10001 shown in FIG. 54 whose display portionhas a display device with the pixel configuration of the invention isincorporated in the prefabricated bath unit 10002, so that a bather canview the display panel 10001. The display panel 10001 has a function ofdisplaying information as well as a function of an advertisement oramusement means with the operation of a bather.

The position for setting the display panel whose display portion has adisplay device with the pixel configuration of the invention is notlimited to the sidewall of the prefabricated bath unit 10002 shown inFIG. 54, and thus the display panel can be provided anywhere by changingthe shape of the panel. For example, the display panel can beincorporated in a part of a mirror or a bathtub.

FIG. 55 shows an example where a television set having a large displayportion is provided in a building. FIG. 55 includes a housing 8010, adisplay portion 8011, a remote controlling device 8012 which is anoperating portion, a speaker portion 8013, and the like. A display panelwhose display portion has a display device with the pixel configurationof the invention is applied to the manufacture of the display portion8011. The television set in FIG. 55 is incorporated in a building as awall-hanging television set, and can be set without requiring a largespace.

Although this embodiment mode has illustrated a telephone pole as acolumnar object, a prefabricated bath unit, and the like as exemplarystructures, the invention is not limited to these, and can be applied toany structures which can incorporate a display device. By using adisplay device whose display portion has the pixel configuration of theinvention, reduction in size and power consumption of the display devicecan be achieved, and a moving object or a structure having a displaymedium which can operate excellently can be provided.

Note that this embodiment mode can be freely combined with anydescription in other embodiment modes in this specification. Further,parts of the description in this embodiment mode can be combined withone another.

The present application is based on Japanese Priority application No.2006-155472 filed on Jun. 2, 2006 with the Japanese Patent Office, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising a firsttransistor, a second transistor, a third transistor, and a fourthtransistor, wherein a gate and a first terminal of the first transistorare electrically connected to a first wiring, wherein a second terminalof the first transistor is electrically connected to a gate of thefourth transistor, wherein a gate of the second transistor iselectrically connected to a second wiring, wherein a first terminal ofthe second transistor is electrically connected to a fourth wiring,wherein a second terminal of the second transistor is electricallyconnected to the gate of the fourth transistor, wherein a gate of thethird transistor is electrically connected to a third wiring, wherein afirst terminal of the third transistor is electrically connected to thefourth wiring, wherein a second terminal of the third transistor iselectrically connected to the gate of the fourth transistor, wherein afirst terminal of the fourth transistor is electrically connected to thefourth wiring, and wherein a second terminal of the fourth transistor iselectrically connected to a fifth wiring.